Semiconductor device and display device including the semiconductor device

ABSTRACT

In a transistor including an oxide semiconductor film, field-effect mobility and reliability are improved. A semiconductor device includes a gate electrode, an insulating film over the gate electrode, an oxide semiconductor film over the insulating film, and a pair of electrodes over the oxide semiconductor film. The oxide semiconductor film includes a first oxide semiconductor film and a second oxide semiconductor film over the first oxide semiconductor film. The first oxide semiconductor film is formed using In oxide or In—Zn oxide. The second oxide semiconductor film is formed using In-M-Zn oxide (M is Al, Ga, or Y) and includes a region where the number of In atoms is 40% or more and 50% or less and the number of M atoms is 5% or more and 30% or less of the total number of In, M, and Zn atoms.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor film. Another embodiment of thepresent invention relates to a display device including thesemiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a memory device, adriving method thereof, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

As a semiconductor material that can be used in a transistor, an oxidesemiconductor has been attracting attention. For example, PatentDocument 1 discloses a semiconductor device whose field-effect mobility(in some cases, simply referred to as mobility or μFE) is improved bystacking a plurality of oxide semiconductor layers, among which theoxide semiconductor layer serving as a channel contains indium andgallium where the proportion of indium is higher than the proportion ofgallium.

Non-Patent Document 1 discloses that an oxide semiconductor containingindium, gallium, and zinc has a homologous series represented byIn_(1−x)Ga_(1+x)O₃(ZnO)_(m) (x is a number which satisfies −1≤x≤1, and mis a natural number). Furthermore, Non-Patent Document 1 discloses asolid solution range of a homologous series. For example, in the solidsolution range of the homologous series in the case where m=1, x rangesfrom −0.33 to 0.08, and in the solid solution range of the homologousseries in the case where m=2, x ranges from −0.68 to 0.32.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2014-007399

Non-Patent Document

-   [Non-Patent Document 1] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., Vol. 93, 1991, pp. 298-315.

SUMMARY OF THE INVENTION

The field-effect mobility of a transistor that uses an oxidesemiconductor film as a channel region is preferably as high aspossible. However, when the field-effect mobility is increased, thetransistor has a problem with its characteristics, that is, thetransistor tends to be normally-on. Note that “normally-on” means astate in which a channel exists without application of a voltage to agate electrode and a current flows through the transistor.

Furthermore, in a transistor that uses an oxide semiconductor film in achannel region, oxygen vacancies formed in the oxide semiconductor filmadversely affect the transistor characteristics. For example, oxygenvacancies formed in the oxide semiconductor film are bonded to hydrogento serve as a carrier supply source. The carrier supply source generatedin the oxide semiconductor film causes a change in the electricalcharacteristics, typically, shift in the threshold voltage, of thetransistor including the oxide semiconductor film.

Too many oxygen vacancies in an oxide semiconductor film, for example,shift the threshold voltage of the transistor in the negative direction,causing normally-on characteristics. Thus, it is preferable that achannel region in an oxide semiconductor film especially include fewoxygen vacancies or include oxygen vacancies such that normally-oncharacteristics are not caused.

In view of the foregoing problems, an object of one embodiment of thepresent invention is to improve field-effect mobility and reliability ina transistor including an oxide semiconductor film. Another object ofone embodiment of the present invention is to prevent a change inelectrical characteristics of a transistor including an oxidesemiconductor film and to improve reliability of the transistor. Anotherobject of one embodiment of the present invention is to provide asemiconductor device with low power consumption. Another object of oneembodiment of the present invention is to provide a novel semiconductordevice. Another object of one embodiment of the present invention is toprovide a novel display device.

Note that the description of the above object does not disturb theexistence of other objects. One embodiment of the present invention doesnot necessarily achieve all of these objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film. The semiconductor device includesa gate electrode, an insulating film over the gate electrode, the oxidesemiconductor film over the insulating film, and a pair of electrodesover the oxide semiconductor film. The oxide semiconductor film includesa first oxide semiconductor film and a second oxide semiconductor filmover the first oxide semiconductor film. The first oxide semiconductorfilm is formed using In oxide or In—Zn oxide. The second oxidesemiconductor film is formed using In-M-Zn oxide (M is Al, Ga, or Y).The second oxide semiconductor film includes a region where the numberof In atoms is greater than or equal to 40% and less than or equal to50% of the total number of In, M, and Zn atoms, and a region where thenumber of M atoms is greater than or equal to 5% and less than or equalto 30% of the total number of In, M, and Zn atoms.

In the above embodiment, the second oxide semiconductor film preferablyincludes a region whose sheet resistance is higher than or equal to1×10² Ω/square and lower than 1×10⁶ Ω/square in a region in contact withthe pair of electrodes.

In either of the above embodiments, it is preferable that a peak beobserved at around 2θ=31° not in the first oxide semiconductor film butin the second oxide semiconductor film when a crystal structure of theoxide semiconductor film is measured by XRD analysis.

In any of the above embodiments, the first oxide semiconductor filmpreferably includes a region not containing M.

In any of the above embodiments, when an atomic ratio of In to M to Znis x:y:z and x is 4, the second oxide semiconductor film preferablyincludes a region where y is higher than or equal to 1.5 and lower thanor equal to 2.5 and z is higher than or equal to 2 and lower than orequal to 4. Moreover, the atomic ratio of In to M to Zn is preferably4:2:3 or in the neighborhood thereof.

In any of the above embodiments, when an atomic ratio of In to M to Znis x:y:z and x is 5, the second oxide semiconductor film preferablyincludes a region where y is higher than or equal to 0.5 and lower thanor equal to 1.5 and z is higher than or equal to 5 and lower than orequal to 7. Moreover, the atomic ratio of In to M to Zn is preferably5:1:6 or in the neighborhood thereof.

Another embodiment of the present invention is a display device whichincludes the semiconductor device according to any one of theabove-described embodiments, and a display element. Another embodimentof the present invention is a display module which includes the displaydevice and a touch sensor. Another embodiment of the present inventionis an electronic device which includes the semiconductor deviceaccording to any one of the above-described embodiments, theabove-described display device, or the above-described display module,and an operation key or a battery.

One embodiment of the present invention can improve field-effectmobility and reliability in a transistor including an oxidesemiconductor film. One embodiment of the present invention can preventa change in electrical characteristics of a transistor including anoxide semiconductor film and improve the reliability of the transistor.One embodiment of the present invention can provide a semiconductordevice with low power consumption. One embodiment of the presentinvention can provide a novel semiconductor device. One embodiment ofthe present invention can provide a novel display device.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily have all of these effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views illustrating asemiconductor device;

FIGS. 2A to 2C are a top view and cross-sectional views illustrating asemiconductor device;

FIGS. 3A to 3C are a top view and cross-sectional views illustrating asemiconductor device;

FIGS. 4A to 4C are a top view and cross-sectional views illustrating asemiconductor device;

FIGS. 5A to 5C are a top view and cross-sectional views illustrating asemiconductor device;

FIGS. 6A to 6C are a top view and cross-sectional views illustrating asemiconductor device;

FIGS. 7A to 7C are cross-sectional views illustrating a manufacturingmethod of a semiconductor device;

FIGS. 8A to 8C are cross-sectional views illustrating a manufacturingmethod of a semiconductor device;

FIGS. 9A to 9C are cross-sectional views illustrating a manufacturingmethod of a semiconductor device;

FIGS. 10A and 10B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device;

FIGS. 11A and 11B are schematic views illustrating diffusion paths ofoxygen or excess oxygen diffused into an oxide semiconductor film;

FIGS. 12A to 12C each show a range of the atomic ratio of an oxidesemiconductor film;

FIGS. 13A to 13C are band diagrams of stacked structures of oxidesemiconductor films;

FIG. 14 is a top view of one embodiment of a display device;

FIG. 15 is a cross-sectional view of one embodiment of a display device;

FIG. 16 is a cross-sectional view of one embodiment of a display device;

FIG. 17 is a cross-sectional view of one embodiment of a display device;

FIG. 18 is a cross-sectional view of one embodiment of a display device;

FIG. 19 is a cross-sectional view of one embodiment of a display device;

FIG. 20 is a cross-sectional view of one embodiment of a display device;

FIGS. 21A and 21B illustrate a top surface and a cross section of asemiconductor device;

FIG. 22 illustrates a cross section of a semiconductor device;

FIG. 23 illustrates a structure example of a display panel;

FIG. 24 illustrates a structure example of a display panel;

FIGS. 25A to 25C are a block diagram and circuit diagrams eachillustrating a display device;

FIG. 26 illustrates a display module;

FIGS. 27A to 27E illustrate electronic devices;

FIGS. 28A to 28G illustrate electronic devices;

FIGS. 29A and 29B are a top view and a cross-sectional view illustratingone embodiment of an evaluation sample; and

FIG. 30 shows the sheet resistances of evaluation samples.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented in many different modes, andit will be readily appreciated by those skilled in the art that modesand details thereof can be changed in various ways without departingfrom the spirit and scope of the present invention. Thus, the presentinvention should not be interpreted as being limited to the followingdescription of the embodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over”, “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Further, the positional relation between components is changedas appropriate in accordance with a direction in which the componentsare described. Thus, the positional relation is not limited to thatdescribed with a term used in this specification and can be explainedwith another term as appropriate depending on the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow between thesource and the drain through the channel region. Note that in thisspecification and the like, a channel region refers to a region throughwhich current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

In this specification and the like, the term “parallel” means that theangle formed between two straight lines is greater than or equal to −10°and less than or equal to 10°, and accordingly also covers the casewhere the angle is greater than or equal to −5° and less than or equalto 5°. The term “perpendicular” means that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly also covers the case where the angle is greaterthan or equal to 85° and less than or equal to 95°.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

Unless otherwise specified, the off-state current in this specificationand the like refers to a drain current of a transistor in the off state(also referred to as non-conduction state and cutoff state). Unlessotherwise specified, the off state of an n-channel transistor means thata gate-source voltage V_(gs) is lower than the threshold voltage V_(th),and the off state of a p-channel transistor means that the gate-sourcevoltage V_(gs) is higher than the threshold voltage V_(th). For example,the off-state current of an n-channel transistor sometimes refers to adrain current that flows when the gate-source voltage V_(gs) is lowerthan the threshold voltage V_(th).

The off-state current of a transistor depends on V_(gs) in some cases.Thus, “the off-state current of a transistor is lower than or equal toI” may mean “there is V_(gs) with which the off-state current of thetransistor becomes lower than or equal to I”. Furthermore, “theoff-state current of a transistor” means “the off-state current in anoff state at predetermined V_(gs)”, “the off-state current in an offstate at V_(gs) in a predetermined range”, “the off-state current in anoff state at V_(gs) with which sufficiently reduced off-state current isobtained”, or the like.

As an example, the assumption is made of an n-channel transistor wherethe threshold voltage V_(th) is 0.5 V and the drain current is 1×10⁻⁹ Aat V_(gs) of 0.5 V, 1×10⁻¹³ A at V_(gs) of 0.1 V, 1×10⁻¹⁹ A at V_(gs) of−0.5 V, and 1×10⁻²² A at V_(gs) of −0.8 V. The drain current of thetransistor is 1×10⁻¹⁹ A or lower at V_(gs) of −0.5 V or at V_(gs) in therange of −0.8 V to −0.5 V; therefore, it may be said that the off-statecurrent of the transistor is 1×10⁻¹⁹ A or lower. Since there is V_(gs)at which the drain current of the transistor is 1×10⁻²² A or lower, itmay be said that the off-state current of the transistor is 1×10⁻²² A orlower.

In this specification and the like, the off-state current of atransistor with a channel width W is sometimes represented by a currentvalue in relation to the channel width W or by a current value per givenchannel width (e.g., 1 μm). In the latter case, the off-state currentmay be expressed in the unit with the dimension of current per length(e.g., A/μm).

The off-state current of a transistor depends on temperature in somecases. Unless otherwise specified, the off-state current in thisspecification may be an off-state current at room temperature, 60° C.,85° C., 95° C., or 125° C. Alternatively, the off-state current may bean off-state current at a temperature at which the reliability requiredin a semiconductor device or the like including the transistor isensured or a temperature at which the semiconductor device or the likeincluding the transistor is used (e.g., temperature in the range of 5°C. to 35° C.). The description “an off-state current of a transistor islower than or equal to I” may refer to a situation where there is V_(gs)at which the off-state current of a transistor is lower than or equal toI at room temperature, 60° C., 85° C., 95° C., 125° C., a temperature atwhich the reliability required in a semiconductor device or the likeincluding the transistor is ensured, or a temperature at which thesemiconductor device or the like including the transistor is used (e.g.,temperature in the range of 5° C. to 35° C.).

The off-state current of a transistor depends on a drain-source voltageV_(ds) in some cases. Unless otherwise specified, the off-state currentin this specification may be an off-state current at V_(ds) of 0.1 V,0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V.Alternatively, the off-state current might be an off-state current atV_(ds) at which the required reliability of a semiconductor device orthe like including the transistor is ensured or V_(ds) at which thesemiconductor device or the like including the transistor is used. Thedescription “an off-state current of a transistor is lower than or equalto I” may refer to a situation where there is V_(gs) at which theoff-state current of a transistor is lower than or equal to I at V_(ds)of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V,or 20 V, V_(ds) at which the required reliability of a semiconductordevice or the like including the transistor is ensured, or V_(ds) atwhich the semiconductor device or the like including the transistor isused.

In the above description of off-state current, a drain may be replacedwith a source. That is, the off-state current sometimes refers to acurrent that flows through a source of a transistor in the off state.

In this specification and the like, the term “leakage current” sometimesexpresses the same meaning as off-state current. In this specificationand the like, the off-state current sometimes refers to a current thatflows between a source and a drain when a transistor is off, forexample.

In this specification and the like, the threshold voltage of atransistor refers to a gate voltage (V_(g)) at which a channel is formedin the transistor. Specifically, in a graph where the horizontal axisrepresents the gate voltage (V_(g)) and the vertical axis represents thesquare root of drain current (I_(d)), the threshold voltage of atransistor may refer to a gate voltage (V_(g)) at the intersection ofthe square root of drain current (I_(d)) of 0 (I_(d)=0 A) and anextrapolated straight line that is tangent with the highest inclinationto a plotted curve (V_(g)−√I_(d) characteristics). Alternatively, thethreshold voltage of a transistor may refer to a gate voltage (V_(g)) atwhich the value of I_(d) [A]×L [μm]/W[μm] is 1×10⁻⁹ [A] where L ischannel length and W is channel width.

In this specification and the like, a “semiconductor” can havecharacteristics of an “insulator” when the conductivity is sufficientlylow, for example. Further, a “semiconductor” and an “insulator” cannotbe strictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “insulator” is not clear.Accordingly, a “semiconductor” in this specification and the like can becalled an “insulator” in some cases. Similarly, an “insulator” in thisspecification and the like can be called a “semiconductor” in somecases. An “insulator” in this specification and the like can be called a“semi-insulator” in some cases.

In this specification and the like, a “semiconductor” can havecharacteristics of a “conductor” when the conductivity is sufficientlyhigh, for example. Further, a “semiconductor” and a “conductor” cannotbe strictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “conductor” is not clear.Accordingly, a “semiconductor” in this specification and the like can becalled a “conductor” in some cases. Similarly, a “conductor” in thisspecification and the like can be called a “semiconductor” in somecases.

In this specification and the like, an impurity in a semiconductorrefers to an element that is not a main component of a semiconductorfilm. For example, an element with a concentration of lower than 0.1atomic % is an impurity. If a semiconductor contains an impurity, thedensity of states (DOS) may be formed therein, the carrier mobility maybe decreased, or the crystallinity may be decreased, for example. In thecase where the semiconductor includes an oxide semiconductor, examplesof the impurity which changes the characteristics of the semiconductorinclude Group 1 elements, Group 2 elements, Group 13 elements, Group 14elements, Group 15 elements, and transition metals other than the maincomponents; specific examples are hydrogen (also included in water),lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. Whenthe semiconductor is an oxide semiconductor, oxygen vacancies may beformed by entry of impurities such as hydrogen, for example.Furthermore, in the case where the semiconductor includes silicon,examples of the impurity which changes the characteristics of thesemiconductor include oxygen, Group 1 elements except oxygen andhydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention and a manufacturing method thereof will be describedwith reference to FIGS. 1A to 1C, FIGS. 2A to 2C, FIGS. 3A to 3C, FIGS.4A to 4C, FIGS. 5A to 5C, FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to8C, FIGS. 9A to 9C, FIGS. 10A and 10B, and FIGS. 11A and 11B.

<1-1. Structural Example 1 of Semiconductor Device>

FIG. 1A is a plan view of a transistor 100 that is a semiconductordevice of one embodiment of the present invention. FIG. 1B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 1A,and FIG. 1C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 1A. Note that in FIG. 1A, some components of thetransistor 100 (e.g., an insulating film serving as a gate insulatingfilm) are not illustrated to avoid complexity. Furthermore, thedirection of the dashed dotted line X1-X2 may be referred to as achannel length direction, and the direction of the dashed dotted lineY1-Y2 may be referred to as a channel width direction. As in FIG. 1A,some components are not illustrated in some cases in plan views oftransistors described below.

The transistor 100 includes a conductive film 104 over a substrate 102,an insulating film 106 over the substrate 102 and the conductive film104, an oxide semiconductor film 108 over the insulating film 106, aconductive film 112 a over the oxide semiconductor film 108, and aconductive film 112 b over the oxide semiconductor film 108.Furthermore, an insulating film 114, an insulating film 116 over theinsulating film 114, and an insulating film 118 over the insulating film116 are formed over the transistor 100, specifically over the oxidesemiconductor film 108, the conductive film 112 a, and the conductivefilm 112 b.

Note that the transistor 100 is what is called a channel-etchedtransistor.

Furthermore, the oxide semiconductor film 108 includes an oxidesemiconductor film 108_1 over the insulating film 106 and an oxidesemiconductor film 108_2 over the oxide semiconductor film 108_1. Notethat the oxide semiconductor film 108_1 includes In oxide or In—Znoxide. In addition, the oxide semiconductor film 108_2 includes In-M-Znoxide (M is Al, Ga, or Y).

The oxide semiconductor film 108_2 includes a region where the number ofIn atoms is greater than or equal to 40% and less than or equal to 50%of the total number of In, M, and Zn atoms, and a region where thenumber of M atoms is greater than or equal to 5% and less than or equalto 30% of the total number of In, M, and Zn atoms. The oxidesemiconductor film 108_2 including the above regions can have highcrystallinity and a high carrier density.

Specifically, the atomic ratio of In to M to Zn in the oxidesemiconductor film 108_2 is preferably 4:2:3 or in the neighborhoodthereof or 5:1:6 or in the neighborhood thereof. Here, “4:2:3 or in theneighborhood thereof” means that, in the case where the atomic ratio ofIn to M to Zn is x:y:z and x is 4, y is higher than or equal to 1.5 andlower than or equal to 2.5 and z is higher than or equal to 2 and lowerthan or equal to 4. Moreover, “5:1:6 or in the neighborhood thereof”means that, in the case where the atomic ratio of In to M to Zn is x:y:zand x is 5, y is higher than or equal to 0.5 and lower than or equal to1.5 and z is higher than or equal to 5 and lower than or equal to 7.

The oxide semiconductor film 108_2 preferably includes a region whosesheet resistance is higher than or equal to 1×10² Ω/sq. and lower than1×10⁶ Ω/sq. in a region in contact with the conductive films 112 a and112 b. When the oxide semiconductor film 108_2 includes such a region,the contact resistance between the oxide semiconductor film 108_2 andthe conductive films 112 a and 112 b can be reduced.

In particular, the oxide semiconductor film 108_1 is preferably formedusing In—Zn oxide. The In—Zn oxide can be formed using an oxide targetwith the atomic ratio of In:Zn=2:3. The atomic ratio of the oxidesemiconductor film 108_1 is preferably In:Zn=2:3 or in the neighborhoodthereof. In addition, the oxide semiconductor film 108_1 preferablyincludes a region not containing M (e.g., Ga), which is contained in theoxide semiconductor film 108_2.

When the oxide semiconductor film 108_1 contains Ga, formation of oxygenvacancies in the oxide semiconductor film 108_1 can be suppressedbecause Ga is strongly bonded to oxygen. Accordingly, the stability ofthe transistor including the oxide semiconductor film 108_1 can beincreased. On the other hand, when the oxide semiconductor film 108_1contains Ga, the transistor including the oxide semiconductor film 108_1may have reduced on-state current and reduced field-effect mobility.Therefore, in order to increase the on-state current and field-effectmobility of the transistor, the structure in which the oxidesemiconductor film 108_1 does not contain Ga is favorable.

Moreover, the oxide semiconductor film 108_1 may contain one or moreelements selected from Sn, W, and Hf, in addition to In or Zn. For theoxide semiconductor film 108_1, typically, In—Sn oxide (also referred toas ITO), In—Sn—Zn oxide, In—Hf oxide, In—Hf—Zn oxide, In—W oxide,In—W—Zn oxide, or the like can be used.

Note that Sn, W, and Hf are more strongly bonded to oxygen than In andZn are. Thus, when the oxide semiconductor film 108_1 contains one ormore elements selected from Sn, W, and Hf, the elements, instead of Ga,can suppress formation of oxygen vacancies. Moreover, the valences ofSn, W, and Hf are higher than those of In and Ga. Specifically, Sn andHf have a valence of 4 and W has a valence of 4 or 6 while In and Gahave a valence of 3. With the use of an element whose valence is higherthan those of In and Ga for the oxide semiconductor film 108_1, thiselement may serve as a donor source and may increase the carrier densityof the oxide semiconductor film 108_1. As described above, when theoxide semiconductor film 108_1 contains an element whose valence ishigher than those of In and Ga, formation of oxygen vacancies can besuppressed and the on-state current and field-effect mobility of thetransistor can be increased.

Furthermore, the oxide semiconductor film 108_1 may be formed using Inoxide, In—Zn oxide, In—Sn oxide, In—Sn—Zn oxide, In—Hf oxide, In—Hf—Znoxide, In—W oxide, or In—W—Zn oxide and contain Si. When the oxidesemiconductor film 108_1 contains Si, formation of oxygen vacancies thatcan be formed in the oxide semiconductor film 108_1 can be furthersuppressed. Note that when the oxide semiconductor film 108_1 contains alarge amount of Si, for example, 10 atomic % or more of Si, the densityof defect states in the oxide semiconductor film 108_1 is increased insome cases. Therefore, when the oxide semiconductor film 108_1 containsSi, the content of Si is preferably less than 10 atomic % and morepreferably less than 5 atomic %. For the oxide semiconductor film 108_1containing Si, typically, In—Si oxide, In—Zn—Si oxide, In—Sn—Si oxide(also referred to as ITSO), or the like can be used.

When the oxide semiconductor film 108_1 has the above structure, thetransistor 100 can have high field-effect mobility. Specifically, thefield-effect mobility of the transistor 100 can exceed 50 cm²/Vs,preferably exceed 100 cm²/Vs.

The use of the transistor with high field-effect mobility in a gatedriver that generates a gate signal allows a display device to have anarrow frame, for example. The use of the transistor with highfield-effect mobility in a source driver (particularly in ademultiplexer connected to an output terminal of a shift registerincluded in a source driver) that is included in a display device andsupplies a signal from a signal line can reduce the number of wiringsconnected to the display device.

The crystal structure of the oxide semiconductor film 108_1 is notparticularly limited. The oxide semiconductor film 108_1 may have eitheror both of a single crystal structure and a non-single crystalstructure.

The non-single crystal structure includes, for example, a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) described later, apolycrystalline structure, a microcrystalline structure, and anamorphous structure. Moreover, a bixbyite crystal structure, a layeredcrystal structure, and the like are given as the crystal structure. Amixed crystal structure including a bixbyite crystal structure and alayered crystal structure may be used.

The oxide semiconductor film 108_2 favorably includes a layered crystalstructure, in particular, a crystal structure having c-axis alignment.In other words, the oxide semiconductor film 108_2 favorably includes aCAAC-OS.

It is favorable that the oxide semiconductor film 108_1 includes anamorphous structure or a microcrystalline structure and the oxidesemiconductor film 108_2 includes a crystal structure having c-axisalignment, for example. That is, the oxide semiconductor film 108_1includes a region having lower crystallinity than the oxidesemiconductor film 108_2. Note that the crystallinity of the oxidesemiconductor film 108 can be determined by analysis by X-raydiffraction (XRD) or with a transmission electron microscope (TEM).

For example, when the crystal structure of the oxide semiconductor film108 is measured by XRD analysis, a peak is observed at around 2θ=31° notin the oxide semiconductor film 108_1 but in the oxide semiconductorfilm 108_2.

In the case where the oxide semiconductor film 108_1 has a region havinglow crystallinity, the following effects can be achieved.

First, oxygen vacancies that might be formed in the oxide semiconductorfilm 108 will be described.

Oxygen vacancies formed in the oxide semiconductor film 108 adverselyaffect the transistor characteristics and therefore cause a problem. Forexample, oxygen vacancies formed in the oxide semiconductor film 108 arebonded to hydrogen to serve as a carrier supply source. The carriersupply source generated in the oxide semiconductor film 108 causes achange in the electrical characteristics, typically, shift in thethreshold voltage, of the transistor 100 including the oxidesemiconductor film 108. Therefore, it is preferable that the amount ofoxygen vacancies in the oxide semiconductor film 108 be as small aspossible.

In view of this, one embodiment of the present invention is a structurein which insulating films in the vicinity of the oxide semiconductorfilm 108, specifically the insulating films 114 and 116 formed over theoxide semiconductor film 108, include excess oxygen. Oxygen or excessoxygen is transferred from the insulating films 114 and 116 to the oxidesemiconductor film 108, whereby the oxygen vacancies in the oxidesemiconductor film can be reduced.

Here, the path of oxygen or excess oxygen diffused into the oxidesemiconductor film 108 will be described with reference to FIGS. 11A and11B. FIGS. 11A and 11B are schematic views illustrating the diffusionpaths of oxygen or excess oxygen diffused into the oxide semiconductorfilm 108. FIG. 11A is the schematic view in the channel length directionand FIG. 11B is the schematic view in the channel width direction.

Oxygen or excess oxygen of the insulating films 114 and 116 is diffusedinto the oxide semiconductor film 108_1 from above, i.e., through theoxide semiconductor film 108_2 (Route 1 in FIGS. 11A and 11B).

Alternatively, oxygen or excess oxygen of the insulating films 114 and116 is diffused into the oxide semiconductor film 108 through the sidesurfaces of the oxide semiconductor film 108_1 and the oxidesemiconductor film 108_2 (Route 2 in FIG. 11B).

For example, diffusion of oxygen or excess oxygen by Route 1 shown inFIGS. 11A and 11B is sometimes prevented when the oxide semiconductorfilm 108_2 has high crystallinity. In contrast, oxygen or excess oxygencan be diffused into the oxide semiconductor film 108_1 and the oxidesemiconductor film 108_2 through the side surfaces of the oxidesemiconductor film 108_1 and the oxide semiconductor film 108_2 by Route2 shown in FIG. 11B.

Since the oxide semiconductor film 108_1 includes a region having lowercrystallinity than the oxide semiconductor film 108_2, the region servesas a diffusion path of excess oxygen, so that excess oxygen can also bediffused into the oxide semiconductor film 108_2 having highercrystallinity than the oxide semiconductor film 108_1 by Route 2 shownin FIG. 11B. Although not shown in FIGS. 11A and 11B, when theinsulating film 106 contains oxygen or excess oxygen, the oxygen orexcess oxygen might also be diffused from the insulating film 106 intothe oxide semiconductor film 108.

As described above, a stacked-layer structure that includes the oxidesemiconductor films having different crystal structures is formed in asemiconductor device of one embodiment of the present invention and theregion having low crystallinity serves as a diffusion path of excessoxygen, whereby the semiconductor device can be highly reliable.

Note that in the case where the oxide semiconductor film 108 consistsonly of an oxide semiconductor film having low crystallinity, thereliability might be lowered because of attachment or entry ofimpurities (e.g., hydrogen or moisture) to the back channel side of theoxide semiconductor film, i.e., a region corresponding to the oxidesemiconductor film 108_2.

Impurities such as hydrogen or moisture entering the oxide semiconductorfilm 108 adversely affect the transistor characteristics and thereforecause a problem.

Therefore, it is preferable that the amount of impurities such ashydrogen or moisture in the oxide semiconductor film 108 be as small aspossible.

In view of the above, in one embodiment of the present invention, theoxide semiconductor film over the oxide semiconductor film has highercrystallinity to inhibit entry of impurities to the oxide semiconductorfilm 108. In particular, the higher crystallinity of the oxidesemiconductor film 108_2 can inhibit damage at the time of processingthe conductive films 112 a and 112 b. The surface of the oxidesemiconductor film 108, i.e., the surface of the oxide semiconductorfilm 108_2 is exposed to an etchant or an etching gas at the time ofprocessing the conductive films 112 a and 112 b. However, since theoxide semiconductor film 108_2 includes a region having highcrystallinity and thus has higher etching resistance than the oxidesemiconductor film 108_1 having lower crystallinity, it serves as anetching stopper.

Note that it is preferable to use, as the oxide semiconductor film 108,an oxide semiconductor film in which the impurity concentration is lowand the density of defect states is low, in which case the transistorcan have more excellent electrical characteristics. Here, the state inwhich the impurity concentration is low and the density of defect statesis low (the amount of oxygen vacancies is small) is referred to as“highly purified intrinsic” or “substantially highly purifiedintrinsic”. Typical examples of impurities contained in the oxidesemiconductor film include water and hydrogen. In this specification andthe like, reducing or removing water and hydrogen from the oxidesemiconductor film is referred to as dehydration or dehydrogenation insome cases. Moreover, adding oxygen to the oxide semiconductor film isreferred to as oxygen addition in some cases, and a state in whichoxygen in excess of the stoichiometric composition is contained isreferred to as an oxygen-excess state in some cases.

A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has few carrier generation sources, and thuscan have a low carrier density. Thus, a transistor in which a channelregion is formed in the oxide semiconductor film rarely has a negativethreshold voltage (is rarely normally-on). A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasa low density of defect states and accordingly has a low density of trapstates in some cases. Furthermore, the highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has anextremely low off-state current; even when an element has a channelwidth W of 1×10⁶ μm and a channel length L of 10 μm, the off-statecurrent can be less than or equal to the measurement limit of asemiconductor parameter analyzer, that is, less than or equal to 1×10⁻¹³A, at a voltage (drain voltage) between a source electrode and a drainelectrode of from 1 V to 10 V.

By including a region having lower crystallinity than the oxidesemiconductor film 108_2, the oxide semiconductor film 108_1 sometimeshas a high carrier density. Since the oxide semiconductor film 108_1 isformed using In oxide or In—Zn oxide, the oxide semiconductor film 108_1can have a high carrier density.

When the oxide semiconductor film 108_1 has a high carrier density, theFermi level is sometimes high relative to the conduction band of theoxide semiconductor film 108_1. This lowers the conduction band minimumof the oxide semiconductor film 108_1, so that the energy differencebetween the conduction band minimum of the oxide semiconductor film108_1 and the trap level, which might be formed in a gate insulatingfilm (here, the insulating film 106), is increased in some cases. Theincrease of the energy difference can reduce trap of charges in the gateinsulating film and reduce variation in the threshold voltage of thetransistor, in some cases. In addition, when the oxide semiconductorfilm 108_1 has a high carrier density, the oxide semiconductor film 108can have high field-effect mobility.

In the transistor 100 illustrated in FIGS. 1A to 1C, the insulating film106 functions as a gate insulating film of the transistor 100, and theinsulating films 114, 116, and 118 function as protective insulatingfilms of the transistor 100. Furthermore, in the transistor 100, theconductive film 104 functions as a gate electrode, the conductive film112 a functions as a source electrode, and the conductive film 112 bfunctions as a drain electrode. Note that in this specification and thelike, in some cases, the insulating film 106 is referred to as a firstinsulating film, the insulating films 114 and 116 are collectivelyreferred to as a second insulating film, and the insulating film 118 isreferred to as a third insulating film.

<1-2. Components of Semiconductor Device>

Next, components of the semiconductor device in this embodiment will bedescribed in detail.

[Substrate]

There is no particular limitation on a material and the like of thesubstrate 102 as long as the material has heat resistance high enough towithstand at least heat treatment to be performed later. For example, aglass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like may be used as the substrate 102. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon or silicon carbide, a compoundsemiconductor substrate of silicon germanium, an SOI substrate, or thelike can be used, or any of these substrates provided with asemiconductor element may be used as the substrate 102. In the casewhere a glass substrate is used as the substrate 102, a glass substratehaving any of the following sizes can be used: the 6th generation (1500mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation(2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10thgeneration (2950 mm×3400 mm). Thus, a large-sized display device can befabricated.

Alternatively, a flexible substrate may be used as the substrate 102,and the transistor 100 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 102 and the transistor 100. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate 102 and transferredonto another substrate. In such a case, the transistor 100 can betransferred to a substrate having low heat resistance or a flexiblesubstrate as well.

[Conductive Film]

The conductive film 104 functioning as a gate electrode and theconductive films 112 a and 112 b functioning as a source electrode and adrain electrode can each be formed using a metal element selected fromchromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc(Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W),manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloyincluding any of these metal elements as its component; an alloyincluding a combination of any of these metal elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b can be formedusing an oxide conductor or an oxide semiconductor, such as an oxideincluding indium and tin (In—Sn oxide), an oxide including indium andtungsten (In—W oxide), an oxide including indium, tungsten, and zinc(In—W—Zn oxide), an oxide including indium and titanium (In—Ti oxide),an oxide including indium, titanium, and tin (In—Ti—Sn oxide), an oxideincluding indium and zinc (In—Zn oxide), an oxide including indium, tin,and silicon (In—Sn—Si oxide), or an oxide including indium, gallium, andzinc (In—Ga—Zn oxide).

Here, an oxide conductor is described. In this specification and thelike, an oxide conductor may be referred to as OC. For example, oxygenvacancies are formed in an oxide semiconductor, and then hydrogen isadded to the oxygen vacancies, so that a donor level is formed in thevicinity of the conduction band. This increases the conductivity of theoxide semiconductor; accordingly, the oxide semiconductor becomes aconductor. The oxide semiconductor having become a conductor can bereferred to as an oxide conductor. Oxide semiconductors generallytransmit visible light because of their large energy gap. Since an oxideconductor is an oxide semiconductor having a donor level in the vicinityof the conduction band, the influence of absorption due to the donorlevel is small in an oxide conductor, and an oxide conductor has avisible light transmitting property comparable to that of an oxidesemiconductor.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive films 104, 112 a, and 112 b. The use of a Cu—X alloyfilm results in lower manufacturing costs because the film can beprocessed by wet etching.

Among the above-mentioned metal elements, any one or more elementsselected from copper, titanium, tungsten, tantalum, and molybdenum arefavorably included in the conductive films 112 a and 112 b. Inparticular, a tantalum nitride film is favorably used for the conductivefilms 112 a and 112 b. A tantalum nitride film has conductivity and ahigh barrier property against copper or hydrogen. Because a tantalumnitride film releases little hydrogen from itself, it can be mostfavorably used as the conductive film in contact with the oxidesemiconductor film 108 or the conductive film in the vicinity of theoxide semiconductor film 108. It is favorable to use a copper film forthe conductive films 112 a and 112 b because the resistance of theconductive films 112 a and 112 b can be reduced.

The conductive films 112 a and 112 b can be formed by electrolessplating. As a material that can be deposited by electroless plating, forexample, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag,and Pd can be used. In particular, it is favorable to use Cu or Agbecause the resistance of the conductive film can be reduced.

[Insulating Film Functioning as Gate Insulating Film]

As the insulating film 106 functioning as a gate insulating film of thetransistor 100, an insulating layer including at least one of thefollowing films formed by a plasma enhanced chemical vapor deposition(PECVD) method, a sputtering method, or the like can be used: a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, a hafnium oxide film, anyttrium oxide film, a zirconium oxide film, a gallium oxide film, atantalum oxide film, a magnesium oxide film, a lanthanum oxide film, acerium oxide film, and a neodymium oxide film. Note that the insulatingfilm 106 may have a layered structure of two, three or more layers.

The insulating film 106 that is in contact with the oxide semiconductorfilm 108 functioning as a channel region of the transistor 100 ispreferably an oxide insulating film and preferably includes a regionincluding oxygen in excess of the stoichiometric composition(oxygen-excess region). In other words, the insulating film 106 is aninsulating film capable of releasing oxygen. In order to provide theoxygen-excess region in the insulating film 106, the insulating film 106is formed in an oxygen atmosphere, or the deposited insulating film 106is subjected to heat treatment in an oxygen atmosphere, for example.

In the case where hafnium oxide is used for the insulating film 106, thefollowing effect is attained. Hafnium oxide has higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, theinsulating film 106 using hafnium oxide can have a larger thickness thanthe insulating film 106 using silicon oxide, so that leakage current dueto tunnel current can be low. That is, it is possible to achieve atransistor with a low off-state current. Moreover, hafnium oxide havinga crystal structure has a higher dielectric constant than hafnium oxidehaving an amorphous structure. Therefore, it is preferable to usehafnium oxide having a crystal structure, in order to provide atransistor with a low off-state current. Examples of the crystalstructure include a monoclinic crystal structure and a cubic crystalstructure. Note that one embodiment of the present invention is notlimited to the above examples.

In this embodiment, a layered film of a silicon nitride film and asilicon oxide film is formed as the insulating film 106. The siliconnitride film has a higher dielectric constant than a silicon oxide filmand needs a larger thickness for capacitance equivalent to that of thesilicon oxide film. Thus, when the silicon nitride film is included inthe gate insulating film of the transistor 100, the thickness of theinsulating film can be increased. This makes it possible to reduce adecrease in withstand voltage of the transistor 100 and furthermore toincrease the withstand voltage, thereby reducing electrostatic dischargedamage to the transistor 100.

[Oxide Semiconductor Film]

The oxide semiconductor film 108 can be formed using the materialsdescribed above.

In the case where the oxide semiconductor film 108_1 is formed usingIn—Zn oxide, it is preferable that the atomic ratio of metal elements ofa sputtering target used for forming a film of the In—Zn oxide satisfyIn≥Zn. Examples of the atomic ratio of the metal elements in asputtering target include In:Zn=1:1 and In:Zn=4:1. However, a sputteringtarget is not limited to these examples, a sputtering target satisfyingIn<Zn may be used for forming the oxide semiconductor film 108_1.Examples of the atomic ratio of the metal elements of a sputteringtarget include In:Zn=2:3.

In the case where the oxide semiconductor film 108_2 is formed usingIn-M-Zn oxide, it is preferable that the atomic ratio of metal elementsof a sputtering target used for forming the In-M-Zn oxide satisfy In>MThe atomic ratio of metal elements in such a sputtering target is, forexample, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, In:M:Zn=5:1:6,In:M:Zn=5:1:7, In:M:Zn=5:1:8, In:M:Zn=6:1:6, or In:M:Zn=5:2:5.

In the case where the oxide semiconductor film 108_2 is formed usingIn-M-Zn oxide, it is preferable to use a target includingpolycrystalline In-M-Zn oxide as the sputtering target. The use of thetarget including polycrystalline In-M-Zn oxide facilitates formation ofthe oxide semiconductor film 108_2 having crystallinity. Note that theatomic ratio of metal elements in the formed oxide semiconductor film108_2 varies from the above atomic ratios of metal elements of thesputtering targets in a range of ±40%. For example, when a sputteringtarget with the atomic ratio of In to Ga to Zn of 4:2:4.1 is used, theatomic ratio of In to Ga to Zn in the formed oxide semiconductor film108 may be 4:2:3 or in the neighborhood thereof.

The energy gap of the oxide semiconductor film 108_2 is 2 eV or more,preferably 2.5 eV or more. With the use of an oxide semiconductor havingsuch a wide energy gap, the off-state current of the transistor 100 canbe reduced.

[Insulating Film 1 Functioning as Protective Insulating Film]

The insulating films 114 and 116 function as protective insulating filmsfor the transistor 100. In addition, the insulating films 114 and 116each have a function of supplying oxygen to the oxide semiconductor film108. That is, the insulating films 114 and 116 contain oxygen. Theinsulating film 114 is an insulating film that allows oxygen to passtherethrough. Note that the insulating film 114 also functions as a filmthat relieves damage to the oxide semiconductor film 108 at the time offorming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the insulating film 114.

In addition, it is preferable that the number of defects in theinsulating film 114 be small and typically, the spin densitycorresponding to a signal that appears at g=2.001 due to a dangling bondof silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spinresonance (ESR) measurement. This is because if the density of defectsin the insulating film 114 is high, oxygen is bonded to the defects andthe property of transmitting oxygen of the insulating film 114 islowered.

Note that all oxygen entering the insulating film 114 from the outsidedoes not move to the outside of the insulating film 114 and some oxygenremains in the insulating film 114. Furthermore, movement of oxygenoccurs in the insulating film 114 in some cases in such a manner thatoxygen enters the insulating film 114 and oxygen included in theinsulating film 114 moves to the outside of the insulating film 114.When an oxide insulating film that can transmit oxygen is formed as theinsulating film 114, oxygen released from the insulating film 116provided over the insulating film 114 can be moved to the oxidesemiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxideinsulating film having a low density of states due to nitrogen oxide.Note that the density of states due to nitrogen oxide can be formedbetween the energy of the valence band maximum (E_(v) _(_) _(os)) andthe energy of the conduction band minimum (E_(c) _(_) _(os)) of theoxide semiconductor film. A silicon oxynitride film that releases lessnitrogen oxide, an aluminum oxynitride film that releases less nitrogenoxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide isa film which releases ammonia more than nitrogen oxide in thermaldesorption spectroscopy (TDS) analysis; the amount of released ammoniais typically greater than or equal to 1×10¹⁸ cm⁻³ and less than or equalto 5×10¹⁹ cm⁻³. Note that the amount of released ammonia is the amountof ammonia released by heat treatment with which the surface temperatureof a film becomes higher than or equal to 50° C. and lower than or equalto 650° C., preferably higher than or equal to 50° C. and lower than orequal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2),typically NO₂ or NO, forms levels in the insulating film 114, forexample. The level is positioned in the energy gap of the oxidesemiconductor film 108. Therefore, when nitrogen oxide is diffused intothe interface between the insulating film 114 and the oxidesemiconductor film 108, an electron is in some cases trapped by thelevel on the insulating film 114 side. As a result, the trapped electronremains in the vicinity of the interface between the insulating film 114and the oxide semiconductor film 108; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide included in the insulating film 114 reacts with ammoniaincluded in the insulating film 116 in heat treatment, nitrogen oxideincluded in the insulating film 114 is reduced. Therefore, an electronis hardly trapped at the interface between the insulating film 114 andthe oxide semiconductor film 108.

By using such an oxide insulating film, the insulating film 114 canreduce the shift in the threshold voltage of the transistor, which leadsto a smaller change in the electrical characteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film114, by heat treatment of a manufacturing process of the transistor,typically heat treatment at a temperature higher than or equal to 300°C. and lower than 350° C., a first signal that appears at a g-factor ofgreater than or equal to 2.037 and less than or equal to 2.039, a secondsignal that appears at a g-factor of greater than or equal to 2.001 andless than or equal to 2.003, and a third signal that appears at ag-factor of greater than or equal to 1.964 and less than or equal to1.966 are observed. The split width of the first and second signals andthe split width of the second and third signals that are obtained by ESRmeasurement using an X-band are each approximately 5 mT. The sum of thespin densities of the first signal that appears at a g-factor of greaterthan or equal to 2.037 and less than or equal to 2.039, the secondsignal that appears at a g-factor of greater than or equal to 2.001 andless than or equal to 2.003, and the third signal that appears at ag-factor of greater than or equal to 1.964 and less than or equal to1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the sum of the spin densities ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 corresponds to the sumof the spin densities of signals attributed to nitrogen oxide (NO_(x); xis greater than 0 and less than or equal to 2, preferably greater thanor equal to 1 and less than or equal to 2). Typical examples of nitrogenoxide include nitrogen monoxide and nitrogen dioxide. In other words,the lower the total spin density of the first signal that appears at ag-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 is, the lower the content of nitrogen oxide inthe oxide insulating film is.

The concentration of nitrogen of the above oxide insulating filmmeasured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above oxide insulating film is formed by a PECVD method at asubstrate temperature higher than or equal to 220° C. and lower than orequal to 350° C. using silane and dinitrogen monoxide, whereby a denseand hard film can be formed.

The insulating film 116 is an oxide insulating film which containsoxygen at a higher proportion than the stoichiometric composition. Partof oxygen is released from the above oxide insulating film by heating.The amount of oxygen released from the oxide insulating film in TDS ismore than or equal to 1.0×10¹⁹ atoms/cm³, preferably more than or equalto 3.0×10²⁰ atoms/cm³. Note that the amount of released oxygen is thetotal amount of oxygen released by heat treatment in a temperature rangeof 50° C. to 650° C. or 50° C. to 550° C. in TDS. In addition, theamount of released oxygen is the total amount of released oxygenconverted into oxygen atoms in TDS.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116be small and typically, the spin density corresponding to a signal thatappears at g=2.001 due to a dangling bond of silicon be lower than1.5×10¹⁸ spins/cm³, preferably lower than or equal to 1×10¹⁸ spins/cm³by ESR measurement. Note that the insulating film 116 is provided moreapart from the oxide semiconductor film 108 than the insulating film 114is; thus, the insulating film 116 may have higher density of defectsthan the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed usinginsulating films formed of the same kinds of materials; thus, a boundarybetween the insulating films 114 and 116 cannot be clearly observed insome cases. Thus, in this embodiment, the boundary between theinsulating films 114 and 116 is shown by a broken line. Although atwo-layer structure of the insulating films 114 and 116 is described inthis embodiment, the present invention is not limited to this. Forexample, a single-layer structure of only the insulating film 114 or alayered structure of three or more layers may be employed.

[Insulating Film 2 Functioning as Protective Insulating Film]

The insulating film 118 functions as a protective insulating film forthe transistor 100.

The insulating film 118 includes one or both of hydrogen and nitrogen.Alternatively, the insulating film 118 includes nitrogen and silicon.The insulating film 118 has a function of blocking oxygen, hydrogen,water, alkali metal, alkaline earth metal, or the like. The provision ofthe insulating film 118 makes it possible to prevent outward diffusionof oxygen from the oxide semiconductor film 108, outward diffusion ofoxygen included in the insulating films 114 and 116, and entry ofhydrogen, water, or the like into the oxide semiconductor film 108 fromthe outside.

A nitride insulating film, for example, can be used as the insulatingfilm 118. The nitride insulating film is formed using silicon nitride,silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or thelike.

Although the variety of films such as the conductive films, theinsulating films, and the oxide semiconductor films described above canbe formed by a sputtering method or a PECVD method, such films may beformed by another method, e.g., a thermal chemical vapor deposition(CVD) method. A metal organic chemical vapor deposition (MOCVD) methodand an atomic layer deposition (ALD) method can be given as examples ofa thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.In a thermal CVD method, a source gas is introduced into a chamber, thechamber is set at an atmospheric pressure or a reduced pressure, and afilm is deposited on a substrate.

Furthermore, in an ALD method, a source gas is introduced into achamber, the chamber is set at an atmospheric pressure or a reducedpressure, and a film is deposited on a substrate.

<1-3. Structure Example 2 of Semiconductor Device>

Next, variations of the transistor 100 illustrated in FIGS. 1A to 1Cwill be described with reference to FIGS. 2A to 2C, FIGS. 3A to 3C,FIGS. 4A to 4C, FIGS. 5A to 5C, and FIGS. 6A to 6C.

FIG. 2A is a top view of a transistor 100A that is a semiconductordevice of one embodiment of the present invention. FIG. 2B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 2A,and FIG. 2C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 2A.

Note that the transistor 100A illustrated in FIGS. 2A to 2C is what iscalled a channel-protective transistor. Thus, the semiconductor deviceof one embodiment of the present invention can have either thechannel-etched structure or the channel-protective structure.

In the transistor 100A, the insulating films 114 and 116 have an opening141 a and an opening 141 b. The oxide semiconductor film 108 isconnected to the conductive films 112 a and 112 b through the openings141 a and 141 b. Furthermore, the insulating film 118 is formed over theconductive films 112 a and 112 b. The insulating films 114 and 116function as so-called channel protective films. Note that the othercomponents of the transistor 100A are similar to those of the transistor100 described above, and an effect similar to that of the transistor 100can be obtained.

FIG. 3A is a top view of a transistor 100B that is a semiconductordevice of one embodiment of the present invention. FIG. 3B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 3A,and FIG. 3C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 3A.

The transistor 100B includes the conductive film 104 over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104, the oxide semiconductor film 108 over the insulating film 106,the conductive film 112 a over the oxide semiconductor film 108, theconductive film 112 b over the oxide semiconductor film 108, theinsulating film 114 over the oxide semiconductor film 108, theconductive film 112 a, and the conductive film 112 b, the insulatingfilm 116 over the insulating film 114, a conductive film 120 a over theinsulating film 116, a conductive film 120 b over the insulating film116, and the insulating film 118 over the insulating film 116, theconductive film 120 a, and the conductive film 120 b.

The insulating films 114 and 116 have an opening 142 a. The insulatingfilms 106, 114, and 116 have an opening 142 b. The conductive film 120 ais electrically connected to the conductive film 104 through the opening142 b. Furthermore, the conductive film 120 b is electrically connectedto the conductive film 112 b through the opening 142 a.

Note that in the transistor 100B, the insulating film 106 functions as afirst gate insulating film of the transistor 100B, the insulating films114 and 116 function as a second gate insulating film of the transistor100B, and the insulating film 118 functions as a protective insulatingfilm of the transistor 100B. In the transistor 100B, the conductive film104 functions as a first gate electrode, the conductive film 112 afunctions as a source electrode, and the conductive film 112 b functionsas a drain electrode. In the transistor 100B, the conductive film 120 afunctions as a second gate electrode, and the conductive film 120 bfunctions as a pixel electrode of a display device.

As illustrated in FIG. 3C, the conductive film 120 a is electricallyconnected to the conductive film 104 through the opening 142 b.Accordingly, the conductive film 104 and the conductive film 120 a aresupplied with the same potential.

As illustrated in FIG. 3C, the oxide semiconductor film 108 ispositioned so as to face the conductive film 104 and the conductive film120 a, and is sandwiched between the two conductive films functioning asthe gate electrodes. The length in the channel length direction and thelength in the channel width direction of the conductive film 120 a arelonger than the length in the channel length direction and the length inthe channel width direction of the oxide semiconductor film 108,respectively. The whole oxide semiconductor film 108 is covered with theconductive film 120 a with the insulating films 114 and 116 positionedtherebetween.

In other words, the conductive film 104 and the conductive film 120 aare connected to each other in the opening provided in the insulatingfilms 106, 114, and 116, and each include a region positioned outside anedge portion of the oxide semiconductor film 108.

With this structure, the oxide semiconductor film 108 included in thetransistor 100B can be electrically surrounded by electric fields of theconductive films 104 and 120 a. A device structure of a transistor, likethat of the transistor 100B, in which electric fields of a first gateelectrode and a second gate electrode electrically surround an oxidesemiconductor film where a channel region is formed can be referred toas a surrounded channel (s-channel) structure.

Since the transistor 100B has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 108 by the conductive film 104 functioning as a firstgate electrode; therefore, the current drive capability of thetransistor 100B can be improved and high on-state currentcharacteristics can be obtained. Since the on-state current can beincreased, the size of the transistor 100B can be reduced. In addition,since the oxide semiconductor film 108 is surrounded by the conductivefilm 104 functioning as the first gate electrode and the conductive film120 a functioning as the second gate electrode, the mechanical strengthof the oxide semiconductor film 108 can be increased.

Note that for the conductive films 120 a and 120 b, materials similar tothose described as the materials of the above-described conductive films104, 112 a, and 112 b can be used. In particular, oxide conductive films(OC) are preferable as the conductive films 120 a and 120 b. When theconductive films 120 a and 120 b are formed using an oxide conductivefilm, oxygen can be added to the insulating films 114 and 116.

The other components of the transistor 100B are similar to those of thetransistor 100 described above and have similar effects.

FIG. 4A is a top view of a transistor 100C that is a semiconductordevice of one embodiment of the present invention. FIG. 4B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 4A,and FIG. 4C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 4A.

The transistor 100C is different from the above-described transistor100B in that the conductive films 112 a and 112 b each have athree-layer structure.

The conductive film 112 a of the transistor 100C includes a conductivefilm 112 a_1, a conductive film 112 a_2 over the conductive film 112a_1, and a conductive film 112 a_3 over the conductive film 112 a_2. Theconductive film 112 b of the transistor 100C includes a conductive film112 b_1, a conductive film 112 b_2 over the conductive film 112 b_1, anda conductive film 112 b_3 over the conductive film 112 b_2.

For example, it is favorable that the conductive film 112 a_1, theconductive film 112 b_1, the conductive film 112 a_3, and the conductivefilm 112 b_3 contain one or more elements selected from titanium,tungsten, tantalum, molybdenum, indium, gallium, tin, and zinc.Furthermore, it is favorable that the conductive film 112 a_2 and theconductive film 112 b_2 contain one or more elements selected fromcopper, aluminum, and silver.

Specifically, the conductive film 112 a_1, the conductive film 112 b_1,the conductive film 112 a_3, and the conductive film 112 b_3 can beformed using In—Sn oxide or In—Zn oxide and the conductive film 112 a_2and the conductive film 112 b_2 can be formed using copper.

The above structure is favorable because the wiring resistance of theconductive films 112 a and 112 b can be reduced and diffusion of copperinto the oxide semiconductor film 108 can be inhibited. The abovestructure is favorable also because the contact resistance between theconductive film 112 b and the conductive film 120 b can be reduced. Theother components of the transistor 100C are similar to those of thetransistor 100 described above and have similar effects.

FIG. 5A is a top view of a transistor 100D that is a semiconductordevice of one embodiment of the present invention. FIG. 5B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 5A,and FIG. 5C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 5A.

The transistor 100D is different from the above-described transistor100B in that the conductive films 112 a and 112 b each have athree-layer structure. In addition, the transistor 100D is differentfrom the above-described transistor 100C in the shapes of the conductivefilms 112 a and 112 b.

The conductive film 112 a of the transistor 100D includes the conductivefilm 112 a_1, the conductive film 112 a_2 over the conductive film 112a_1, and the conductive film 112 a_3 over the conductive film 112 a_2.The conductive film 112 b of the transistor 100D includes the conductivefilm 112 b_1, the conductive film 112 b_2 over the conductive film 112b_1, and the conductive film 112 b_3 over the conductive film 112 b_2.Note that the conductive film 112 a_1, the conductive film 112 a_2, theconductive film 112 a_3, the conductive film 112 b_1, the conductivefilm 112 b_2, and the conductive film 112 b_3 can be formed using any ofthe above-described materials.

An end portion of the conductive film 112 a_1 has a region locatedoutward from an end portion of the conductive film 112 a_2. Theconductive film 112 a_3 covers a top surface and a side surface of theconductive film 112 a_2 and has a region that is in contact with theconductive film 112 a_1. An end portion of the conductive film 112 b_1has a region located outward from an end portion of the conductive film112 b_2. The conductive film 112 b_3 covers a top surface and a sidesurface of the conductive film 112 b_2 and has a region that is incontact with the conductive film 112 b_1.

The above structure is favorable because the wiring resistance of theconductive films 112 a and 112 b can be reduced and diffusion of copperinto the oxide semiconductor film 108 can be inhibited. Note thatdiffusion of copper can be more favorably inhibited in the transistor100D than in the above-described transistor 100C. The above structure isfavorable also because the contact resistance between the conductivefilm 112 b and the conductive film 120 b can be reduced. The othercomponents of the transistor 100D are similar to those of the transistor100 described above and have similar effects.

FIG. 6A is a top view of a transistor 100E that is a semiconductordevice of one embodiment of the present invention. FIG. 6B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 6A,and FIG. 6C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 6A.

The transistor 100E is different from the transistor 100D describedabove in the position of the conductive films 120 a and 120 b.Specifically, the conductive films 120 a and 120 b of the transistor100E are positioned over the insulating film 118. The other componentsof the transistor 100E are similar to those of the transistor 100Ddescribed above and have similar effects.

The structures of the transistors in this embodiment can be freelycombined with each other.

<1-4. Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of the transistor 100B that is asemiconductor device of one embodiment of the present invention will bedescribed with reference to FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to9C, and FIGS. 10A and 10B.

FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9C, and FIGS. 10A and 10Bare cross-sectional views illustrating a manufacturing method of thesemiconductor device. In each of FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS.9A to 9C, and FIGS. 10A and 10B, the left part is a cross-sectional viewin the channel length direction, and the right part is a cross-sectionalview in the channel width direction.

First, a conductive film is formed over the substrate 102 and processedthrough a lithography process and an etching process, whereby theconductive film 104 functioning as the first gate electrode is formed.Then, the insulating film 106 functioning as the first gate insulatingfilm is formed over the conductive film 104 (see FIG. 7A).

In this embodiment, a glass substrate is used as the substrate 102, andas the conductive film 104 functioning as the first gate electrode, a50-nm-thick titanium film and a 200-nm-thick copper film are each formedby a sputtering method. A 400-nm-thick silicon nitride film and a50-nm-thick silicon oxynitride film as the insulating film 106 areformed by a PECVD method.

Note that the above-described silicon nitride film has a three-layerstructure of a first silicon nitride film, a second silicon nitridefilm, and a third silicon nitride film. An example of the three-layerstructure is as follows.

For example, the first silicon nitride film can be formed to have athickness of 50 nm under the conditions where silane at a flow rate of200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at aflow rate of 100 sccm are supplied as a source gas to a reaction chamberof a PECVD apparatus, the pressure in the reaction chamber is controlledto 100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300nm under the conditions where silane at a flow rate of 200 sccm,nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rateof 2000 sccm are supplied as a source gas to the reaction chamber of thePECVD apparatus, the pressure in the reaction chamber is controlled to100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50nm under the conditions where silane at a flow rate of 200 sccm andnitrogen at a flow rate of 5000 sccm are supplied as a source gas to thereaction chamber of the PECVD apparatus, the pressure in the reactionchamber is controlled to 100 Pa, and a power of 2000 W is supplied usinga 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitridefilm, and the third silicon nitride film can each be formed at asubstrate temperature of lower than or equal to 350° C.

When the silicon nitride film has the above-described three-layerstructure, for example, in the case where a conductive film includingcopper is used as the conductive film 104, the following effect can beobtained.

The first silicon nitride film can inhibit diffusion of copper from theconductive film 104. The second silicon nitride film has a function ofreleasing hydrogen and can improve withstand voltage of the insulatingfilm functioning as a gate insulating film. The third silicon nitridefilm releases a small amount of hydrogen and can inhibit diffusion ofhydrogen released from the second silicon nitride film.

Next, an oxide semiconductor film 108_1_0 is formed over the insulatingfilm 106 (see FIG. 7B).

FIG. 7B is a schematic cross-sectional view illustrating the inside of adeposition apparatus when the oxide semiconductor film 108_1_0 is formedover the insulating film 106. In FIG. 7B, a sputtering apparatus is usedas the deposition apparatus, and a target 191 placed inside thesputtering apparatus and plasma 192 formed under the target 191 areschematically illustrated.

Note that in FIG. 7B, oxygen or excess oxygen added to the insulatingfilm 106 is schematically shown by arrows of broken lines. When anoxygen gas is used in forming the oxide semiconductor film 108_1_0,oxygen can be favorably added to the insulating film 106.

The thickness of the oxide semiconductor film 108_1_0 is greater than orequal to 1 nm and less than or equal to 25 nm, preferably greater thanor equal to 5 nm and less than or equal to 20 nm. The oxidesemiconductor film 108_1_0 is formed using one or both of an inert gas(typically, an argon gas) and an oxygen gas. The percentage of oxygenflow rate in forming the oxide semiconductor film 108_1_0 is higher thanor equal to 0% and lower than 30%, preferably higher than or equal to 5%and lower than or equal to 15%.

When the percentage of oxygen flow rate for forming the oxidesemiconductor film 108_1_0 is in the above range, the oxidesemiconductor film 108_1_0 can have lower crystallinity.

In this embodiment, the oxide semiconductor film 108_1_0 is formed by asputtering method using an In—Zn metal oxide target (In:Zn=2:3 in anatomic ratio). The substrate temperature during the formation of theoxide semiconductor film 108_1_0 is room temperature, and an argon gasat a flow rate of 180 sccm and an oxygen gas at a flow rate of 20 sccmare used as a deposition gas (percentage of oxygen flow rate: 10%).

Next, an oxide semiconductor film 108_2_0 is formed over the oxidesemiconductor film 108_1_0 (see FIG. 7C).

FIG. 7C is a schematic cross-sectional view illustrating the inside of adeposition apparatus when the oxide semiconductor film 108_2_0 is formedover the oxide semiconductor film 108_1_0. In FIG. 7C, a sputteringapparatus is used as the deposition apparatus, and a target 193 placedinside the sputtering apparatus and plasma 194 formed under the target193 are schematically illustrated.

Note that in FIG. 7C, oxygen or excess oxygen added to the oxidesemiconductor film 108_1_0 is schematically shown by arrows of brokenlines. When an oxygen gas is used in forming the oxide semiconductorfilm 108_2_0, oxygen can be favorably added to the oxide semiconductorfilm 108_1_0.

The thickness of the oxide semiconductor film 108_2_0 is greater than orequal to 20 nm and less than or equal to 100 nm, preferably greater thanor equal to 20 nm and less than or equal to 50 nm. Note that when theoxide semiconductor film 108_2_0 is formed, plasma discharge isfavorably performed in an atmosphere containing an oxygen gas. Note thatwhen plasma discharge is performed in an atmosphere containing an oxygengas, oxygen is added into the oxide semiconductor film 108_1_0 overwhich the oxide semiconductor film 108_2_0 is to be formed. Thepercentage of oxygen flow rate in forming the oxide semiconductor film108_2_0 is higher than or equal to 30% and lower than or equal to 100%,preferably higher than or equal to 50% and lower than or equal to 100%,further preferably higher than or equal to 70% and lower than or equalto 100%.

When the percentage of oxygen flow rate for forming the oxidesemiconductor film 108_2_0 is in the above range, the oxidesemiconductor film 108_2_0 can have higher crystallinity.

In this embodiment, the oxide semiconductor film 108_2_0 is formed by asputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1in an atomic ratio). The substrate temperature during the formation ofthe oxide semiconductor film 108_2_0 is room temperature, and an oxygengas at a flow rate of 200 sccm is used as a deposition gas (percentageof oxygen flow rate: 100%).

As described above, the percentage of oxygen flow rate for forming theoxide semiconductor film 108_2_0 is preferably higher than thepercentage of oxygen flow rate for forming the oxide semiconductor film108_1_0. In other words, the oxide semiconductor film 108_1_0 ispreferably formed under a lower oxygen partial pressure than the oxidesemiconductor film 108_2_0.

When the percentage of oxygen flow rate in forming the oxidesemiconductor film 108_1_0 is different from that in forming the oxidesemiconductor film 108_2_0, a layered film having a plurality of kindsof crystallinity can be formed.

The substrate temperature at the time of formation of the oxidesemiconductor film 108_1_0 and the oxide semiconductor film 108_2_0 isset at higher than or equal to room temperature (25° C.) and lower thanor equal to 200° C., preferably higher than or equal to room temperatureand lower than or equal to 130° C. Setting the substrate temperature inthe above range is favorable for large glass substrates (e.g., theabove-described 8th- to 10th-generation glass substrates). Specifically,when the substrate temperature for forming the oxide semiconductor film108_1_0 and the oxide semiconductor film 108_2_0 is set at roomtemperature, bending or distortion of the substrate can be inhibited.

To further increase the crystallinity of the oxide semiconductor film108_2_0, the substrate temperature in forming the oxide semiconductorfilm 108_2_0 is preferably increased (for example, higher than or equalto 100° C. and lower than or equal to 200° C., preferably 130° C.).

In addition, it is more favorable to successively form the oxidesemiconductor film 108_1_0 and the oxide semiconductor film 108_2_0 in avacuum because impurities can be prevented from being caught at theinterfaces.

In addition, increasing the purity of a sputtering gas is necessary. Forexample, as an oxygen gas or an argon gas used as a sputtering gas, agas which is highly purified to have a dew point of −40° C. or lower,preferably −80° C. or lower, further preferably −100° C. or lower, stillfurther preferably −120° C. or lower is used, whereby entry of moistureor the like into the oxide semiconductor film can be minimized as muchas possible.

In the case where the oxide semiconductor film is deposited by asputtering method, a chamber in a sputtering apparatus is preferablyevacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopumpin order to remove water or the like, which serves as an impurity forthe oxide semiconductor film, as much as possible. In particular, thepartial pressure of gas molecules corresponding to H₂O (gas moleculescorresponding to m/z=18) in the chamber in the standby mode of thesputtering apparatus is lower than or equal to 1×10⁻⁴ Pa, preferably5×10⁻⁵ Pa.

Next, the oxide semiconductor film 108_1_0 and the oxide semiconductorfilm 108_2_0 are processed into desired shapes, so that theisland-shaped oxide semiconductor film 108_1 and the island-shaped oxidesemiconductor film 108_2 are formed. In this embodiment, the oxidesemiconductor film 108_1 and the oxide semiconductor film 108_2constitute the island-shaped oxide semiconductor film 108 (see FIG. 8A).

Heat treatment (hereinafter referred to as first heat treatment) isfavorably performed after the oxide semiconductor film 108 is formed. Bythe first heat treatment, water, hydrogen, or the like contained in theoxide semiconductor film 108 can be reduced. The heat treatment for thepurpose of reducing hydrogen, water, and the like may be performedbefore the oxide semiconductor film 108 is processed into an islandshape. Note that the first heat treatment is one kind of treatment forincreasing the purity of the oxide semiconductor film.

The first heat treatment is performed at a temperature of, for example,higher than or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 200° C. and lower than orequal to 450° C., further preferably higher than or equal to 250° C. andlower than or equal to 350° C.

Moreover, an electric furnace, an RTA apparatus, or the like can be usedfor the first heat treatment. With the use of an RTA apparatus, the heattreatment can be performed at a temperature higher than or equal to thestrain point of the substrate if the heating time is short. Therefore,the heat treatment time can be shortened. The first heat treatment maybe performed in an atmosphere of nitrogen, oxygen, ultra-dry air (airwith a water content of 20 ppm or less, preferably 1 ppm or less,further preferably 10 ppb or less), or a rare gas (e.g., argon, helium).It is preferable that hydrogen, water, and the like not be contained inthe nitrogen, oxygen, ultra-dry air, or rare gas. Furthermore, afterheat treatment is performed under a nitrogen atmosphere or a rare gasatmosphere, heat treatment may be additionally performed in an oxygenatmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water,and the like can be released from the oxide semiconductor film andoxygen can be supplied to the oxide semiconductor film at the same time.Consequently, the number of oxygen vacancies in the oxide semiconductorfilm can be reduced.

Next, a conductive film 112 is formed over the insulating film 106 andthe oxide semiconductor film 108 (see FIG. 8B).

In this embodiment, as the conductive film 112, a 30-nm-thick titaniumfilm, a 200-nm-thick copper film, and a 10-nm-thick titanium film areformed in this order by a sputtering method.

Next, the conductive film 112 is processed into a desired shape, so thatthe island-shaped conductive film 112 a and the island-shaped conductivefilm 112 b are formed (see FIG. 8C).

In this embodiment, the conductive film 112 is processed with a wetetching apparatus. Note that the method for processing the conductivefilm 112 is not limited to the above-described method, and a dry etchingapparatus may be used, for example.

After the conductive films 112 a and 112 b are formed, a surface (on theback channel side) of the oxide semiconductor film 108 (specifically,the oxide semiconductor film 108_2) may be cleaned. The cleaning may beperformed, for example, using a chemical solution such as a phosphoricacid. The cleaning using a chemical solution such as a phosphoric acidcan remove impurities (e.g., an element included in the conductive films112 a and 112 b) attached to the surface of the oxide semiconductor film108_2. Note that the cleaning is not necessarily performed; in somecases, the cleaning is not performed.

In the step of forming the conductive films 112 a and 112 b and/or thecleaning step, the thickness of a region of the oxide semiconductor film108 which is not covered with the conductive films 112 a and 112 b mightbe reduced.

Note that in the semiconductor device of one embodiment of the presentinvention, the region not covered with the conductive films 112 a and112 b, i.e., the oxide semiconductor film 108_2 is an oxidesemiconductor film with improved crystallinity. Impurities (inparticular, constituent elements used in the conductive films 112 a and112 b) are not easily diffused into an oxide semiconductor film havinghigh crystallinity. Accordingly, a highly reliable semiconductor devicecan be provided.

Although FIG. 8C illustrates an example in which the surface of theoxide semiconductor film 108 not covered with the conductive films 112 aand 112 b, i.e., the surface of the oxide semiconductor film 108_2 has adepression, one embodiment of the present invention is not limited tothis example and the surface of the oxide semiconductor film 108 notcovered with the conductive films 112 a and 112 b does not necessarilyhave a depression.

Next, the insulating film 114 and the insulating film 116 are formedover the oxide semiconductor film 108 and the conductive films 112 a and112 b (see FIG. 9A).

Note that after the insulating film 114 is formed, the insulating film116 is preferably formed successively without exposure to the air. Whenthe insulating film 116 is formed successively after the formation ofthe insulating film 114 without exposure to the air while at least oneof the flow rate of a source gas, the pressure, high-frequency power,and the substrate temperature is adjusted, the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating films 114 and 116 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film canbe formed by a PECVD method. In that case, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include dinitrogen monoxide and nitrogen dioxide. The flowrate of the oxidizing gas is more than or equal to 20 times and lessthan or equal to 500 times, preferably more than or equal to 40 timesand less than or equal to 100 times, that of the deposition gas.

In this embodiment, a silicon oxynitride film is formed as theinsulating film 114 by a PECVD method under the following conditions:the substrate 102 is held at a temperature of 220° C., silane at a flowrate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm areused as a source gas, the pressure in the treatment chamber is 20 Pa,and a high-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as thepower density) is supplied to a parallel-plate electrode.

As the insulating film 116, a silicon oxide film or a silicon oxynitridefilm is formed under the following conditions: the substrate placed inthe treatment chamber of the PECVD apparatus that is vacuum-evacuated isheld at a temperature of higher than or equal to 180° C. and lower thanor equal to 350° C., the pressure in the treatment chamber is higherthan or equal to 100 Pa and lower than or equal to 250 Pa, preferablyhigher than or equal to 100 Pa and lower than or equal to 200 Pa, withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

As the deposition conditions of the insulating film 116, thehigh-frequency power having the above power density is supplied to thereaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the insulating film 116 becomes higher than that in thestoichiometric composition. In the film formed at a substratetemperature within the above temperature range, the bond between siliconand oxygen is weak, and accordingly, part of oxygen in the film isreleased by heat treatment in a later step. Thus, it is possible to forman oxide insulating film which contains more oxygen than that in thestoichiometric composition and from which part of oxygen is released byheating.

Note that the insulating film 114 functions as a protective film for theoxide semiconductor film 108 in the step of forming the insulating film116. Therefore, the insulating film 116 can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, whenthe flow rate of the deposition gas containing silicon with respect tothe oxidizing gas is increased, the amount of defects in the insulatingfilm 116 can be reduced. Typically, it is possible to form an oxideinsulating film in which the amount of defects is small, i.e., the spindensity of a signal which appears at g=2.001 due to a dangling bond ofsilicon, is lower than 6×10¹⁷ spins/cm³, preferably lower than or equalto 3×10¹⁷ spins/cm³, further preferably lower than or equal to 1.5×10¹⁷spins/cm³ by ESR measurement. As a result, the reliability of thetransistor 100 can be improved.

Heat treatment (hereinafter referred to as second heat treatment) isfavorably performed after the insulating films 114 and 116 are formed.The second heat treatment can reduce nitrogen oxide included in theinsulating films 114 and 116. By the second heat treatment, part ofoxygen contained in the insulating films 114 and 116 can be transferredto the oxide semiconductor film 108, so that the amount of oxygenvacancies included in the oxide semiconductor film 108 can be reduced.

The temperature of the second heat treatment is typically lower than400° C., preferably lower than 375° C., further preferably higher thanor equal to 150° C. and lower than or equal to 350° C. The second heattreatment may be performed in an atmosphere of nitrogen, oxygen,ultra-dry air (air with a water content of less than or equal to 20 ppm,preferably less than or equal to 1 ppm, further preferably less than orequal to 10 ppb), or a rare gas (e.g., argon, helium). It is preferablethat hydrogen, water, and the like not be contained in the nitrogen,oxygen, ultra-dry air, or rare gas. An electric furnace, RTA, or thelike can be used for the heat treatment.

Next, the openings 142 a and 142 b are formed in desired regions in theinsulating films 114 and 116 (see FIG. 9B).

In this embodiment, the openings 142 a and 142 b are formed with a dryetching apparatus. Note that the opening 142 a reaches the conductivefilm 112 b, and the opening 142 b reaches the conductive film 104.

Next, a conductive film 120 is formed over the insulating film 116 (seeFIG. 9C).

FIG. 9C is a schematic cross-sectional view illustrating the inside of adeposition apparatus when the conductive film 120 is formed over theinsulating film 116. In FIG. 9C, a sputtering apparatus is used as thedeposition apparatus, and a target 195 placed inside the sputteringapparatus and plasma 196 formed under the target 195 are schematicallyillustrated.

When the conductive film 120 is formed, plasma discharge is performed inan atmosphere containing an oxygen gas. At this time, oxygen is added tothe insulating film 116 over which the conductive film 120 is to beformed. When the conductive film 120 is formed, an inert gas (e.g., ahelium gas, an argon gas, or a xenon gas) and the oxygen gas may bemixed.

The oxygen gas is mixed at least when the conductive film 120 is formed.The proportion of the oxygen gas in a deposition gas for forming theconductive film 120 is higher than 0% and lower than or equal to 100%,preferably higher than or equal to 10% and lower than or equal to 100%,further preferably higher than or equal to 30% and lower than or equalto 100%.

In FIG. 9C, oxygen or excess oxygen added to the insulating film 116 isschematically shown by arrows of broken lines.

In this embodiment, the conductive film 120 is formed by a sputteringmethod using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in anatomic ratio).

Note that although oxygen is added to the insulating film 116 when theconductive film 120 is formed in this embodiment, the method for addingoxygen is not limited to this example. For example, oxygen may befurther added to the insulating film 116 after the conductive film 120is formed.

As the method for adding oxygen to the insulating film 116, an ITSO filmwith a thickness of 5 nm may be formed using a target of an oxideincluding indium, tin, and silicon (In—Sn—Si oxide, also referred to asITSO) (In₂O₃:SnO₂:SiO₂=85:10:5 in wt %), for example. In that case, thethickness of the ITSO film is greater than or equal to 1 nm and lessthan or equal to 20 nm or greater than or equal to 2 nm and less than orequal to 10 nm, in which case oxygen is favorably transmitted andrelease of oxygen can be inhibited. Then, oxygen is added to theinsulating film 116 through the ITSO film. Oxygen can be added by, forexample, ion doping, ion implantation, or plasma treatment. Byapplication of a bias voltage to the substrate side when oxygen isadded, oxygen can be effectively added to the insulating film 116. Anashing apparatus is used, for example, and the power density of the biasvoltage applied to the substrate side of the ashing apparatus can begreater than or equal to 1 W/cm² and less than or equal to 5 W/cm². Thesubstrate temperature during addition of oxygen is higher than or equalto room temperature and lower than or equal to 300° C., preferablyhigher than or equal to 100° C. and lower than or equal to 250° C.,whereby oxygen can be added efficiently to the insulating film 116.

Next, the conductive film 120 is processed into a desired shape, so thatthe island-shaped conductive film 120 a and the island-shaped conductivefilm 120 b are formed (see FIG. 10A).

In this embodiment, the conductive film 120 is processed with a wetetching apparatus.

Next, the insulating film 118 is formed over the insulating film 116 andthe conductive films 120 a and 120 b (see FIG. 10B).

The insulating film 118 includes one or both of hydrogen and nitrogen.As the insulating film 118, a silicon nitride film is favorably used,for example. The insulating film 118 can be formed by a sputteringmethod or a PECVD method, for example. In the case where the insulatingfilm 118 is formed by a PECVD method, for example, the substratetemperature is lower than 400° C., preferably lower than 375° C., andfurther preferably higher than or equal to 180° C. and lower than orequal to 350° C. The substrate temperature at which the insulating film118 is formed is preferably within the above range because a dense filmcan be formed. Furthermore, when the substrate temperature at which theinsulating film 118 is formed is within the above range, oxygen orexcess oxygen in the insulating films 114 and 116 can be moved to theoxide semiconductor film 108.

In the case where a silicon nitride film is formed by a PECVD method asthe insulating film 118, a deposition gas containing silicon, nitrogen,and ammonia are preferably used as a source gas. A small amount ofammonia compared with the amount of nitrogen is used, whereby ammonia isdissociated in the plasma and activated species are generated. Theactivated species cleave a bond between silicon and hydrogen which areincluded in a deposition gas including silicon and a triple bond betweennitrogen molecules. As a result, a dense silicon nitride film having fewdefects, in which bonds between silicon and nitrogen are promoted andbonds between silicon and hydrogen are few, can be formed. If the amountof ammonia with respect to nitrogen is large, decomposition of adeposition gas including silicon and decomposition of nitrogen are notpromoted, so that a sparse silicon nitride film in which bonds betweensilicon and hydrogen remain and defects are increased is formed.Therefore, in the source gas, the flow rate of nitrogen is set to bepreferably 5 times or more and 50 times or less, further preferably 10times or more and 50 times or less the flow rate of ammonia.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thicksilicon nitride film is formed as the insulating film 118 using silane,nitrogen, and ammonia as a source gas. The flow rate of silane is 50sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate ofammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa,the substrate temperature is 350° C., and high-frequency power of 1000 Wis supplied to a parallel-plate electrode with a 27.12 MHzhigh-frequency power source. The PECVD apparatus is a parallel-platePECVD apparatus in which the electrode area is 6000 cm², and the powerper unit area (power density) into which the supplied power is convertedis 1.7×10⁻¹ W/cm².

In the case where the conductive films 120 a and 120 b are formed usingan In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio),either or both of hydrogen and nitrogen included in the insulating film118 might enter the conductive films 120 a and 120 b. In this case,either or both of hydrogen and nitrogen might be bonded to oxygenvacancies in the conductive films 120 a and 120 b to cause a reductionin the resistance of the conductive films 120 a and 120 b.

After the insulating film 118 is formed, heat treatment similar to thefirst heat treatment or the second heat treatment (hereinafter referredto as third heat treatment) may be performed.

By the third heat treatment, oxygen included in the insulating film 116moves into the oxide semiconductor film 108 to fill the oxygen vacanciesin the oxide semiconductor film 108.

Through the above process, the transistor 100B illustrated in FIGS. 3Ato 3C can be manufactured.

The transistor 100 illustrated in FIGS. 1A to 1C can be manufactured byforming the insulating film 118 after the step of FIG. 9A. Thetransistor 100A illustrated in FIGS. 2A to 2C can be manufactured bychanging the formation order of the conductive films 112 a and 112 b andthe insulating films 114 and 116 and, in addition, adding a step forforming the openings 141 a and 141 b in the insulating films 114 and116.

At least part of this embodiment can be implemented in combination withany of the other embodiments and example described in this specificationas appropriate.

Embodiment 2

In this embodiment, an oxide semiconductor film of one embodiment of thepresent invention will be described with reference to FIGS. 12A to 12Cand FIGS. 13A to 13C.

<2-1. Oxide Semiconductor Film>

The oxide semiconductor film of one embodiment of the present inventionwill be described below.

The oxide semiconductor film of one embodiment of the present inventionincludes a first oxide semiconductor film and a second oxidesemiconductor film over the first oxide semiconductor film.

For the first oxide semiconductor film, In oxide or In—Zn oxide ispreferably used. In particular, In—Zn oxide containing indium and zincis preferable.

For the second oxide semiconductor film, In-M-Zn oxide (M is Al, Ga, orY) is preferably used. Furthermore, one or more elements selected fromboron, titanium, iron, nickel, germanium, zirconium, molybdenum,lanthanum, cerium, neodymium, tantalum, magnesium, and the like may begiven as Min addition to the above elements.

<2-2. Structure of Oxide Semiconductor Film>

An oxide semiconductor film is classified into a single crystal oxidesemiconductor film and a non-single-crystal oxide semiconductor film.Examples of a non-single-crystal oxide semiconductor film include ac-axis-aligned crystalline oxide semiconductor (CAAC-OS), apolycrystalline oxide semiconductor film, a nanocrystalline oxidesemiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS)film, and an amorphous oxide semiconductor film.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in thea-b plane direction, and its crystal structure has distortion. Note thatthe distortion is a portion where the direction of a lattice arrangementchanges between a region with a regular lattice arrangement and anotherregion with a regular lattice arrangement in a region where nanocrystalsare connected.

The shape of the nanocrystal is basically hexagon. However, the shape isnot always a regular hexagon and is a non-regular hexagon in some cases.A pentagonal lattice arrangement, a heptagonal lattice arrangement, andthe like are included in the distortion in some cases. Note that a clearcrystal grain boundary cannot be observed even in the vicinity ofdistortion in the CAAC-OS. That is, formation of a grain boundary isinhibited due to the distortion of lattice arrangement. This is probablybecause the CAAC-OS can tolerate distortion owing to a low density ofthe atomic arrangement in an a-b plane direction, an interatomic bonddistance changed by substitution of a metal element, and the like.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different nanocrystals in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS cannot be distinguished from ana-like OS or an amorphous oxide semiconductor film, depending on theanalysis method.

An a-like OS is an oxide semiconductor film having a structure betweenthe structure of an nc-OS and the structure of an amorphous oxidesemiconductor film. The a-like OS contains a void or a low-densityregion. That is, the a-like OS has low crystallinity as compared withthe nc-OS and the CAAC-OS.

An oxide semiconductor film has various structures which show variousdifferent properties. Two or more of an amorphous oxide semiconductorfilm, a polycrystalline oxide semiconductor film, an a-like OS, annc-OS, and a CAAC-OS may be included in the oxide semiconductor film ofone embodiment of the present invention.

In the case where the first oxide semiconductor film is formed usingIn—Zn oxide, the In—Zn oxide includes a layered crystal structure (alsoreferred to as a layered structure) in which a layer containing indiumand oxygen (hereinafter, In layer) and a layer containing zinc andoxygen (hereinafter, Zn layer) are stacked.

In the case where the first oxide semiconductor film is formed using Inoxide or In—Zn oxide, the In oxide or the In—Zn oxide has a bixbyitecrystal structure.

In the case where the second oxide semiconductor film is formed usingIn-M-Zn oxide, the In-M-Zn oxide includes a layered crystal structure inwhich a layer containing indium and oxygen (hereinafter, In layer) and alayer containing the element M, zinc, and oxygen (hereinafter, (M,Zn)layer) are stacked. Note that indium and the element M can be replacedwith each other and when the element M in the (M,Zn) layer is replacedwith indium, the layer can also be referred to as an (In,M,Zn) layer.Also, when indium in the In layer is replaced with the element M, thelayer can be referred to as an (In,M) layer.

<2-3. Atomic Ratio of Oxide Semiconductor Film>

Next, preferred ranges of the atomic ratio of the elements contained inthe oxide semiconductor film of one embodiment of the present inventionwill be described with reference to FIGS. 12A to 12C. Note that theproportion of oxygen atoms is not shown in FIGS. 12A to 12C. The termsof the atomic ratio of indium, the element M, and zinc contained in theoxide semiconductor film are denoted by [In], [M], and [Zn],respectively.

In FIGS. 12A to 12C, broken lines indicate a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):1, where −1≤α≤1, a line where the atomicratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):5.

In addition, dash-dotted lines indicate a line where the atomic ratio[In]:[M]:[Zn] is 5:1:β (β≥0), a line where the atomic ratio[In]:[M]:[Zn] is 2:1:β, a line where the atomic ratio [In]:[M]:[Zn] is1:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a linewhere the atomic ratio [In]:[M]:[Zn] is 1:3:β, and a line where theatomic ratio [In]:[M]:[Zn] is 1:4:β.

Furthermore, an oxide semiconductor film with the atomic ratio of[In]:[M]:[Zn]=0:2:1 or a neighborhood thereof in FIGS. 12A to 12C tendsto have a spinel crystal structure.

A plurality of phases (e.g., two phases or three phases) exists in theoxide semiconductor film in some cases. For example, with the atomicratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinelcrystal structure and a layered crystal structure are likely to exist.In addition, with the atomic ratio [In]:[M]:[Zn] that is close to 1:0:0,two phases of a bixbyite crystal structure and a layered crystalstructure are likely to exist. In the case where a plurality of phasesexist in the oxide semiconductor film, a grain boundary might be formedbetween different crystal structures.

A region A in FIG. 12A represents examples of the preferred ranges ofthe atomic ratio of indium, the element M, and zinc contained in anoxide semiconductor film.

Note that the region A includes atomic ratios on a line connecting apoint where the atomic ratio [In]:[M]:[Zn] is 1:0:0 and a point wherethe atomic ratio [In]:[M]:[Zn] is 0:0:1.

The oxide semiconductor film containing indium in a higher proportioncan have high carrier mobility (electron mobility). Therefore, an oxidesemiconductor film containing indium in a higher proportion has highercarrier mobility than that of an oxide semiconductor film containingindium in a lower proportion.

In contrast, when the indium content and the zinc content in an oxidesemiconductor film become lower, the carrier mobility becomes lower.Thus, with the atomic ratio of [In]:[M]:[Zn]=0:1:0 and the neighborhoodthereof (e.g., a region C in FIG. 12C), insulation performance becomesbetter.

Accordingly, the oxide semiconductor film of one embodiment of thepresent invention preferably has an atomic ratio represented by theregion A in FIG. 12A. With an atomic ratio represented by the region A,the oxide semiconductor film can have high carrier mobility.

In particular, in the case where the first oxide semiconductor film isformed using In oxide or In—Zn oxide, the atomic ratios [In]:[M]:[Zn]represented by white circles in FIG. 12A are preferably 1:0:0, 4:0:1,1:0:1, and 2:0:3.

It is particularly favorable that the first oxide semiconductor film hasan atomic ratio on a line connecting a point where the atomic ratio[In]:[M]:[Zn] is 4:0:1 and a point where the atomic ratio [In]:[M]:[Zn]is 2:0:3, in which case carrier mobility can be improved.

In the case where the second oxide semiconductor film contains In-M-Znoxide, the atomic ratio is preferably within the range of a region B inFIG. 12B, which is in the region A in FIG. 12A. An oxide semiconductorfilm with an atomic ratio in the region B in FIG. 12B is excellentbecause the oxide semiconductor film easily becomes a CAAC-OS and hashigh carrier mobility.

The CAAC-OS is an oxide semiconductor having high crystallinity. Incontrast, in the CAAC-OS, a reduction in electron mobility due to thegrain boundary is less likely to occur because a clear grain boundarycannot be observed. In addition, entry of impurities, formation ofdefects, or the like might decrease the crystallinity of an oxidesemiconductor film. This means that the CAAC-OS is an oxidesemiconductor having small amounts of impurities and defects (e.g.,oxygen vacancies). Thus, an oxide semiconductor film including a CAAC-OSis physically stable. Therefore, the oxide semiconductor film includinga CAAC-OS is resistant to heat and has high reliability.

The above-described oxide semiconductor film including a CAAC-OS withhigh reliability can be favorably used as an oxide semiconductor filmpositioned on the back channel side of a transistor, for example.

Note that the region B includes the atomic ratio of [In]:[M]:[Zn]=4:2:3to 4.1 and the neighborhood thereof. The neighborhood includes theatomic ratio of [In]:[M]:[Zn]=5:3:4, for example. Note that the region Bincludes the atomic ratio of [In]:[M]:[Zn]=5:1:6 and the neighborhoodthereof and the atomic ratio of [In]:[M]:[Zn]=5:1:7 and the neighborhoodthereof.

Note that the property of an oxide semiconductor film is not uniquelydetermined by an atomic ratio. Even with the same atomic ratio, theproperty of the oxide semiconductor film might be different depending ona formation condition. For example, in the case where the oxidesemiconductor film is formed with a sputtering apparatus, a film havingan atomic ratio deviated from the atomic ratio of a target is formed. Inaddition, [Zn] in the film might be smaller than [Zn] in the targetdepending on the substrate temperature in deposition. Thus, theillustrated regions each represent an atomic ratio with which an oxidesemiconductor film tends to have specific properties, and boundaries ofthe regions A to C are not clear.

<2-4. Transistor Including Oxide Semiconductor Film>

Next, the case where the oxide semiconductor film is used for atransistor will be described.

Note that when the oxide semiconductor film is used for a transistor,carrier scattering or the like at a grain boundary can be reduced; thus,the transistor can have high field-effect mobility. In addition, thetransistor can have high reliability.

An oxide semiconductor film with low carrier density is preferably usedfor the transistor. In order to reduce the carrier density of the oxidesemiconductor film, the impurity concentration in the oxidesemiconductor film is reduced so that the density of defect states canbe reduced. In this specification and the like, a state with a lowimpurity concentration and a low density of defect states is referred toas a highly purified intrinsic or substantially highly purifiedintrinsic state. For example, an oxide semiconductor film whose carrierdensity is lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³,further preferably lower than 1×10¹⁰/cm³, and greater than or equal to1×10⁻⁹/cm³ is used.

A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has a low density of defect states andaccordingly has a low density of trap states in some cases.

Charges trapped by the trap states in the oxide semiconductor film takea long time to be released and may behave like fixed charges. Thus, thetransistor whose channel region is formed in the oxide semiconductorfilm having a high density of trap states has unstable electricalcharacteristics in some cases.

To obtain stable electrical characteristics of the transistor, it iseffective to reduce the concentration of impurities in the oxidesemiconductor film. In addition, to reduce the concentration ofimpurities in the oxide semiconductor film, the concentration ofimpurities in a film that is adjacent to the oxide semiconductor film ispreferably reduced. Examples of impurities include hydrogen, nitrogen,alkali metal, alkaline earth metal, iron, nickel, and silicon.

<2-5. Impurities in Oxide Semiconductor Film>

Next, the influence of impurities in the oxide semiconductor film willbe described.

When silicon or carbon that is one of Group 14 elements is contained inthe oxide semiconductor film, defect states are formed. Thus, theconcentration of silicon or carbon in the oxide semiconductor film andaround an interface with the oxide semiconductor film (measured bysecondary ion mass spectrometry (SIMS)) is set lower than or equal to2×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁷atoms/cm³.

When the oxide semiconductor film contains alkali metal or alkalineearth metal, defect states are formed and carriers are generated, insome cases. Thus, a transistor including an oxide semiconductor filmwhich contains alkali metal or alkaline earth metal is likely to benormally-on. Therefore, it is preferable to reduce the concentration ofalkali metal or alkaline earth metal in the oxide semiconductor film.Specifically, the concentration of alkali metal or alkaline earth metalin the oxide semiconductor film, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³.

When the oxide semiconductor film contains nitrogen, the oxidesemiconductor film easily becomes n-type by generation of electronsserving as carriers and an increase of carrier density. Thus, atransistor including an oxide semiconductor film which contains nitrogenis likely to be normally-on. For this reason, nitrogen in the oxidesemiconductor film is preferably reduced as much as possible; thenitrogen concentration in the oxide semiconductor film measured by SIMSis set, for example, lower than 5×10¹⁹ atoms/cm³, preferably lower thanor equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to1×10¹⁸ atoms/cm³, and still further preferably lower than or equal to5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and thus causes an oxygen vacancy,in some cases. Due to entry of hydrogen into the oxygen vacancy, anelectron serving as a carrier is generated in some cases. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalatom causes generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor film that contains hydrogenis likely to be normally-on. Accordingly, it is preferable that hydrogenin the oxide semiconductor film be reduced as much as possible.Specifically, the hydrogen concentration in the oxide semiconductor filmmeasured by SIMS is set lower than 1×10²⁰ atoms/cm³, preferably lowerthan 1×10¹⁹ atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³,and still further preferably lower than 1×10¹⁸ atoms/cm³.

When an oxide semiconductor film with sufficiently reduced impurityconcentration is used for a channel region in a transistor, thetransistor can have stable electrical characteristics.

<2-6. Band Diagram>

Next, the case where the above-described oxide semiconductor film has atwo-layer structure or a three-layer structure will be described.

A band diagram of a stacked-layer structure of an oxide semiconductorfilm S1, an oxide semiconductor film S2, and an oxide semiconductor filmS3 and insulating films that are in contact with the stacked-layerstructure, a band diagram of a stacked-layer structure of the oxidesemiconductor films S2 and S3 and insulating films that are in contactwith the stacked-layer structure, and a band diagram of a stacked-layerstructure of the oxide semiconductor films S1 and S2 and insulatingfilms that are in contact with the stacked-layer structure are describedwith reference to FIGS. 13A to 13C.

FIG. 13A is an example of a band diagram of a layered structureincluding an insulating film I1, the oxide semiconductor film S1, theoxide semiconductor film S2, the oxide semiconductor film S3, and aninsulating film 12 in a thickness direction. FIG. 13B is an example of aband diagram of a layered structure including the insulating film I1,the oxide semiconductor film S2, the oxide semiconductor film S3, andthe insulating film 12 in a thickness direction. FIG. 13C is an exampleof a band diagram of a layered structure including the insulating filmI1, the oxide semiconductor film S1, the oxide semiconductor film S2,and the insulating film 12 in a thickness direction. Note that for easyunderstanding, the band diagrams show the conduction band minimum (Ec)of each of the insulating film I1, the oxide semiconductor film S1, theoxide semiconductor film S2, the oxide semiconductor film S3, and theinsulating film 12.

The conduction band minimum of each of the oxide semiconductor films S1and S3 is closer to the vacuum level than that of the oxidesemiconductor film S2. Typically, a difference between the conductionband minimum of the oxide semiconductor film S2 and the conduction bandminimum of each of the oxide semiconductor films S1 and S3 is preferablygreater than or equal to 0.15 eV or greater than or equal to 0.5 eV, andless than or equal to 2 eV or less than or equal to 1 eV. That is, thedifference between the electron affinity of each of the oxidesemiconductor films S1 and S3 and the electron affinity of the oxidesemiconductor film S2 is greater than or equal to 0.15 eV or greaterthan or equal to 0.5 eV, and less than or equal to 2 eV or less than orequal to 1 eV.

As illustrated in FIGS. 13A to 13C, the conduction band minimum of eachof the oxide semiconductor films S1 to S3 is gradually varied. In otherwords, the conduction band minimum is continuously varied orcontinuously connected. To obtain such a band diagram, the density ofdefect states in a mixed layer formed at an interface between the oxidesemiconductor films S1 and S2 or an interface between the oxidesemiconductor films S2 and S3 is preferably made low.

Specifically, when the oxide semiconductor films S1 and S2 or the oxidesemiconductor films S2 and S3 contain the same element (as a maincomponent) in addition to oxygen, a mixed layer with a low density ofdefect states can be formed. For example, in the case where the oxidesemiconductor film S2 is an In—Ga—Zn oxide semiconductor film, it ispreferable to use an In—Ga—Zn oxide semiconductor film, a Ga—Zn oxidesemiconductor film, gallium oxide, or the like as each of the oxidesemiconductor films S1 and S3.

At this time, the oxide semiconductor film S2 serves as a main carrierpath. Since the density of defect states at the interface between theoxide semiconductor films S1 and S2 and the interface between the oxidesemiconductor films S2 and S3 can be made low, the influence ofinterface scattering on carrier conduction is small, and a high on-statecurrent can be obtained.

When an electron is trapped in a trap state, the trapped electronbehaves like fixed charge; thus, the threshold voltage of the transistoris shifted in a positive direction. The oxide semiconductor films S1 andS3 can make the trap state apart from the oxide semiconductor film S2.This structure can prevent the positive shift of the threshold voltageof the transistor.

A material whose conductivity is sufficiently lower than that of theoxide semiconductor film S2 is used for the oxide semiconductor films S1and S3. In that case, the oxide semiconductor film S2, the interfacebetween the oxide semiconductor films S1 and S2, and the interfacebetween the oxide semiconductor films S2 and S3 mainly function as achannel region. For example, an oxide semiconductor film with highinsulation performance and an atomic ratio represented by the region Cin FIG. 12C can be used as the oxide semiconductor films S1 and S3. Notethat the region C illustrated in FIG. 12C represents the atomic ratio[In]:[M]:[Zn] of 0:1:0, 1:3:2, and 1:3:4 and the neighborhood thereof.

In the case where an oxide semiconductor film with an atomic ratiorepresented by the region A is used as the oxide semiconductor film S2,it is favorable to use an oxide semiconductor film with [M]/[In] ofgreater than or equal to 1, preferably greater than or equal to 2, aseach of the oxide semiconductor films S1 and S3. In addition, it isfavorable to use an oxide semiconductor film with sufficiently highinsulation performance and [M]/([Zn]+[In]) of greater than or equal to 1as the oxide semiconductor film S3.

Note that the structures described in this embodiment can be used incombination with any of the structures described in the otherembodiments and example as appropriate.

Embodiment 3

In this embodiment, an example of a display device including thetransistor described in the above embodiment will be described withreference to FIG. 14, FIG. 15, FIG. 16, FIG. 18, FIG. 19, and FIG. 20.

FIG. 14 is a top view of an example of a display device. A displaydevice 700 illustrated in FIG. 14 includes a pixel portion 702 providedover a first substrate 701, a source driver circuit portion 704 and agate driver circuit portion 706 provided over the first substrate 701, asealant 712 provided to surround the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706, anda second substrate 705 provided to face the first substrate 701. Thefirst substrate 701 and the second substrate 705 are sealed with thesealant 712. That is, the pixel portion 702, the source driver circuitportion 704, and the gate driver circuit portion 706 are sealed with thefirst substrate 701, the sealant 712, and the second substrate 705.Although not illustrated in FIG. 14, a display element is providedbetween the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 electrically connected to the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706 isprovided in a region different from the region which is surrounded bythe sealant 712 and positioned over the first substrate 701.Furthermore, an FPC 716 is connected to the FPC terminal portion 708,and a variety of signals and the like are supplied to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706 through the FPC 716. Furthermore, a signal line 710 isconnected to the pixel portion 702, the source driver circuit portion704, the gate driver circuit portion 706, and the FPC terminal portion708. The variety of signals and the like are applied to the pixelportion 702, the source driver circuit portion 704, the gate drivercircuit portion 706, and the FPC terminal portion 708 via the signalline 710 from the FPC 716.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. An example of the display device 700 in which thesource driver circuit portion 704 and the gate driver circuit portion706 are formed over the first substrate 701 where the pixel portion 702is also formed is described; however, the structure is not limitedthereto. For example, only the gate driver circuit portion 706 may beformed over the first substrate 701 or only the source driver circuitportion 704 may be formed over the first substrate 701. In this case, asubstrate where a source driver circuit, a gate driver circuit, or thelike is formed (e.g., a driver-circuit substrate formed using asingle-crystal semiconductor film or a polycrystalline semiconductorfilm) may be formed on the first substrate 701. Note that there is noparticular limitation on the method of connecting a separately prepareddriver circuit substrate, and a chip on glass (COG) method, a wirebonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a plurality of transistors. As the plurality of transistors, anyof the transistors that are described in the above embodiments can beused.

The display device 700 can include any of a variety of elements.Examples of the elements include an electroluminescent (EL) element(e.g., an EL element including organic and inorganic materials, anorganic EL element, an inorganic EL element, or an LED), alight-emitting transistor element (a transistor that emits lightdepending on current), an electron-emissive element, a liquid crystalelement, an electronic ink element, an electrophoretic element, anelectrowetting element, a plasma display panel (PDP), a display usingmicro electro mechanical systems (MEMS) (e.g., a grating light valve(GLV), a digital micromirror device (DMD), a digital micro shutter (DMS)element, or an interferometric modulation (IMOD) element), apiezoelectric ceramic display, and the like.

In addition, examples of display devices having EL elements include anEL display. Examples of display devices including electron-emissiveelements include a field emission display (FED) and an SED-type flatpanel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including an electronicink element or electrophoretic elements is electronic paper. In the casewhere a transflective liquid crystal display or a reflective liquidcrystal display is achieved, some of or all of pixel electrodes functionas reflective electrodes. For example, some or all of pixel electrodesare formed to include aluminum, silver, or the like. In such a case, amemory circuit such as an SRAM can be provided under the reflectiveelectrodes, leading to lower power consumption.

As a display method in the display device 700, a progressive method, aninterlace method, or the like can be employed. Furthermore, colorelements controlled in a pixel at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of the R pixel,the G pixel, the B pixel, and a W (white) pixel may be included.Alternatively, a color element may be composed of two colors among R, G,and B as in PenTile layout. The two colors may differ among colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Furthermore, the size of a displayregion may be different depending on respective dots of the colorcomponents. Embodiments of the disclosed invention are not limited to adisplay device for color display; the disclosed invention can also beapplied to a display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used toobtain a full-color display device in which white light (W) is used fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp). For example, a red (R) coloring layer, agreen (G) coloring layer, a blue (B) coloring layer, and a yellow (Y)coloring layer can be combined as appropriate. With the use of thecoloring layer, high color reproducibility can be obtained as comparedwith the case without the coloring layer. Here, by providing a regionwith a coloring layer and a region without a coloring layer, white lightin the region without the coloring layer may be directly utilized fordisplay. By partly providing the region without a coloring layer, adecrease in the luminance of a bright image due to the coloring layercan be suppressed, and approximately 20% to 30% of power consumption canbe reduced in some cases. In the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light in their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption may be further reduced as compared with the case of using acoloring layer.

As a coloring system, any of the following systems may be used: theabove-described color filter system in which part of white light isconverted into red light, green light, and blue light through colorfilters; a three-color system in which red light, green light, and bluelight are used; and a color conversion system or a quantum dot system inwhich part of blue light is converted into red light or green light.

In this embodiment, a structure including a liquid crystal element as adisplay element and a structure including an EL element as a displayelement will be described with reference to FIG. 15 and FIG. 17. FIG. 15is a cross-sectional view taken along a dashed dotted line Q-R in FIG.14 and illustrates the structure including a liquid crystal element as adisplay element. FIG. 17 is a cross-sectional view taken along a dasheddotted line Q-R in FIG. 14 and illustrates the structure including an ELelement as a display element.

Portions common to FIG. 15 and FIG. 17 will be described first, andthen, different portions will be described.

<3-1. Portions Common to Display Devices>

The display device 700 in FIG. 15 and FIG. 17 includes a lead wiringportion 711, the pixel portion 702, the source driver circuit portion704, and the FPC terminal portion 708. The lead wiring portion 711includes the signal line 710. The pixel portion 702 includes atransistor 750 and a capacitor 790. The source driver circuit portion704 includes a transistor 752.

The transistor 750 and the transistor 752 each have a structure similarto that of the transistor 100D described above. Note that the transistor750 and the transistor 752 may each have the structure of any of theother transistors described in the above embodiments.

The transistor used in this embodiment includes an oxide semiconductorfilm that is highly purified and in which formation of oxygen vacanciesis inhibited. The transistor can have low off-state current.Accordingly, an electrical signal such as an image signal can be heldfor a long time, and a long writing interval can be set in an on state.Accordingly, the frequency of refresh operation can be reduced, whichsuppresses power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high-speed operation.For example, in a liquid crystal display device that includes such atransistor capable of high-speed operation, a switching transistor in apixel portion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, no additional semiconductor deviceformed using a silicon wafer or the like is needed as a driver circuit;therefore, the number of components of the semiconductor device can bereduced. In addition, by using the transistor capable of high-speedoperation in the pixel portion, a high-quality image can be provided.

The capacitor 790 includes a lower electrode and an upper electrode. Thelower electrode is formed through a step of processing the conductivefilm to be the conductive film functioning as a first gate electrode ofthe transistor 750. The upper electrode is formed through a step ofprocessing the conductive film to be the conductive film functioning asa source electrode or a drain electrode of the transistor 750. Betweenthe lower electrode and the upper electrode, an insulating film formedthrough a step of forming the insulating film to be the insulating filmfunctioning as a first gate insulating film of the transistor 750 isprovided. That is, the capacitor 790 has a stacked-layer structure inwhich an insulating film functioning as a dielectric film is positionedbetween the pair of electrodes.

In FIG. 15 and FIG. 17, a planarization insulating film 770 is providedover the transistor 750, the transistor 752, and the capacitor 790.

The planarization insulating film 770 can be formed using aheat-resistant organic material, such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating film770 may be formed by stacking a plurality of insulating films formedfrom these materials. Alternatively, a structure without theplanarization insulating film 770 may be employed.

Although FIG. 15 and FIG. 17 each illustrate an example in which thetransistor 750 included in the pixel portion 702 and the transistor 752included in the source driver circuit portion 704 have the samestructure, one embodiment of the present invention is not limitedthereto. For example, the pixel portion 702 and the source drivercircuit portion 704 may include different transistors. Specifically, astructure in which a staggered transistor is used in the pixel portion702 and the inverted staggered transistor described in Embodiment 1 isused in the source driver circuit portion 704, or a structure in whichthe inverted staggered transistor described in Embodiment 1 is used inthe pixel portion 702 and a staggered transistor is used in the sourcedriver circuit portion 704 may be employed. Note that the term “sourcedriver circuit portion 704” can be replaced by the term “gate drivercircuit portion”.

The signal line 710 is formed through the same process as the conductivefilms functioning as source electrodes and drain electrodes of thetransistors 750 and 752. In the case where the signal line 710 is formedusing a material containing a copper element, signal delay or the likedue to wiring resistance is reduced, which enables display on a largescreen.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed through the same process as theconductive films functioning as source electrodes and drain electrodesof the transistors 750 and 752. The connection electrode 760 iselectrically connected to a terminal included in the FPC 716 through theanisotropic conductive film 780.

Glass substrates can be used as the first substrate 701 and the secondsubstrate 705, for example. As the first substrate 701 and the secondsubstrate 705, flexible substrates may also be used. An example of theflexible substrate is a plastic substrate.

A structure 778 is provided between the first substrate 701 and thesecond substrate 705. The structure 778 is a columnar spacer obtained byselective etching of an insulating film and is provided to control thedistance (cell gap) between the first substrate 701 and the secondsubstrate 705. Alternatively, a spherical spacer may also be used as thestructure 778.

A light-blocking film 738 functioning as a black matrix, a coloring film736 functioning as a color filter, and an insulating film 734 in contactwith the light-blocking film 738 and the coloring film 736 are providedon the second substrate 705 side.

<3-2. Structure Example of Display Device Including Liquid CrystalElement>

The display device 700 in FIG. 15 includes a liquid crystal element 775.The liquid crystal element 775 includes a conductive film 772, aconductive film 774, and a liquid crystal layer 776. The conductive film774 is provided on the second substrate 705 side and functions as acounter electrode. The display device 700 in FIG. 15 can display animage in such a manner that transmission or non-transmission of light iscontrolled by the alignment state in the liquid crystal layer 776 thatis changed depending on the voltage applied between the conductive film772 and the conductive film 774.

The conductive film 772 is electrically connected to the conductive filmfunctioning as the source electrode or the drain electrode of thetransistor 750. The conductive film 772 is formed over the planarizationinsulating film 770 and functions as a pixel electrode, that is, oneelectrode of the display element. The conductive film 772 has a functionof a reflective electrode. The display device 700 in FIG. 15 is what iscalled a reflective color liquid crystal display device in whichexternal light is reflected by the conductive film 772 to display animage through the coloring film 736.

A conductive film that transmits visible light or a conductive film thatreflects visible light can be used as the conductive film 772. Forexample, a material containing an element selected from indium (In),zinc (Zn), and tin (Sn) is preferably used for the conductive film thattransmits visible light. For example, a material containing aluminum orsilver is preferably used for the conductive film that reflects visiblelight. In this embodiment, a conductive film that reflects visible lightis used as the conductive film 772.

Although FIG. 15 illustrates an example in which the conductive film 772is connected to the conductive film functioning as the drain electrodeof the transistor 750, one embodiment of the present invention is notlimited to this example. For example, as illustrated in FIG. 16, theconductive film 772 may be electrically connected to the conductive filmfunctioning as the drain electrode of the transistor 750 through aconductive film 777 functioning as a connection electrode. Note that theconductive film 777 is formed by a step of processing the conductivefilm to be the conductive film functioning as a second gate electrode ofthe transistor 750 and thus can be formed without adding a manufacturingstep.

Note that the display device 700 is not limited to the example in FIG.15, which illustrates a reflective color liquid crystal display device,and may be a transmissive color liquid crystal display device in which aconductive film that transmits visible light is used as the conductivefilm 772. Alternatively, the display device 700 may be what is called atransflective color liquid crystal display device in which a reflectivecolor liquid crystal display device and a transmissive color liquidcrystal display device are combined.

FIG. 18 illustrates an example of a transmissive color liquid crystaldisplay device. FIG. 18 is a cross-sectional view taken along a dasheddotted line Q-R in FIG. 14 and illustrates the structure including aliquid crystal element as a display element. The display device 700illustrated in FIG. 18 is an example of employing a horizontal electricfield mode (e.g., an FFS mode) as a driving mode of the liquid crystalelement. In the structure illustrated in FIG. 18, an insulating film 773is provided over the conductive film 772 functioning as the pixelelectrode, and the conductive film 774 is provided over the insulatingfilm 773. In such a structure, the conductive film 774 functions as acommon electrode, and an electric field generated between the conductivefilm 772 and the conductive film 774 through the insulating film 773 cancontrol the alignment state in the liquid crystal layer 776.

Although not illustrated in FIG. 15 and FIG. 18, the conductive film 772and/or the conductive film 774 may be provided with an alignment film ona side in contact with the liquid crystal layer 776. Although notillustrated in FIG. 15 and FIG. 18, an optical member (opticalsubstrate) or the like, such as a polarizing member, a retardationmember, or an anti-reflection member, may be provided as appropriate.For example, circular polarization may be obtained by using a polarizingsubstrate and a retardation substrate. In addition, a backlight, asidelight, or the like may be used as a light source.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. These liquid crystal materials exhibit acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

In the case where a horizontal electric field mode is employed, a liquidcrystal exhibiting a blue phase for which an alignment film isunnecessary may be used. The blue phase is one of liquid crystal phases,which is generated just before a cholesteric phase changes into anisotropic phase when the temperature of a cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition containing a liquid crystal exhibiting a blue phase and achiral material has a short response time and optical isotropy, whicheliminates the need for an alignment process. An alignment film does notneed to be provided, and thus, rubbing treatment is not necessary;accordingly, electrostatic discharge damage caused by the rubbingtreatment can be prevented, and defects and damage of a liquid crystaldisplay device in the manufacturing process can be reduced. Moreover,the liquid crystal material that exhibits a blue phase has small viewingangle dependence.

In the case where a liquid crystal element is used as a display element,a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a fringefield switching (FFS) mode, an axially symmetric aligned micro-cell(ASM) mode, an optical compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as avertical alignment (VA) mode transmissive liquid crystal display devicemay also be used. There are some examples of a vertical alignment mode;for example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, and an ASV mode, or the like can beemployed.

<3-3. Display Device Including Light-Emitting Element>

The display device 700 illustrated in FIG. 17 includes a light-emittingelement 782. The light-emitting element 782 includes a conductive film772, an EL layer 786, and a conductive film 788. The display device 700illustrated in FIG. 17 can display an image by utilizing light emissionfrom the EL layer 786 of the light-emitting element 782. Note that theEL layer 786 contains an organic compound or an inorganic compound suchas a quantum dot.

Examples of materials that can be used for an organic compound include afluorescent material and a phosphorescent material. Examples ofmaterials that can be used for a quantum dot include a colloidal quantumdot material, an alloyed quantum dot material, a core-shell quantum dotmaterial, and a core quantum dot material. A material containingelements belonging to Groups 12 and 16, elements belonging to Groups 13and 15, or elements belonging to Groups 14 and 16, may be used.Alternatively, a quantum dot material containing an element such ascadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P),indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), oraluminum (Al) may be used.

In the display device 700 in FIG. 17, the insulating film 730 isprovided over the planarization insulating film 770 and the conductivefilm 772. The insulating film 730 covers part of the conductive film772. Note that the light-emitting element 782 has a top-emissionstructure. Thus, the conductive film 788 has a light-transmittingproperty and transmits light emitted from the EL layer 786. Although thetop-emission structure is described as an example in this embodiment,the structure is not limited thereto. For example, a bottom-emissionstructure in which light is emitted to the conductive film 772 side or adual-emission structure in which light is emitted to both the conductivefilm 772 side and the conductive film 788 side may also be employed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided in the leadwiring portion 711 and the source driver circuit portion 704 to overlapwith the insulating film 730. The coloring film 736 and thelight-blocking film 738 are covered with the insulating film 734. Aspace between the light-emitting element 782 and the insulating film 734is filled with a sealing film 732. The structure of the display device700 is not limited to the example in FIG. 17, in which the coloring film736 is provided. For example, a structure without the coloring film 736may also be employed in the case where the EL layer 786 is formed byseparate coloring.

<3-4. Structure Example of Display Device Provided with Input/OutputDevice>

An input/output device may be provided in the display device 700illustrated in FIG. 17 and FIG. 18. As an example of the input/outputdevice, a touch panel or the like can be given.

FIG. 19 illustrates a structure in which the display device 700 in FIG.17 includes a touch panel 791, and FIG. 20 illustrates a structure inwhich the display device 700 in FIG. 18 includes the touch panel 791.

FIG. 19 is a cross-sectional view of the structure in which the touchpanel 791 is provided in the display device 700 illustrated in FIG. 17,and FIG. 20 is a cross-sectional view of the structure in which thetouch panel 791 is provided in the display device 700 illustrated inFIG. 18.

First, the touch panel 791 illustrated in FIG. 19 and FIG. 20 will bedescribed below.

The touch panel 791 illustrated in FIG. 19 and FIG. 20 is what is calledan in-cell touch panel provided between the substrate 705 and thecoloring film 736. The touch panel 791 is formed on the substrate 705side before the light-blocking film 738 and the coloring film 736 areformed.

Note that the touch panel 791 includes the light-blocking film 738, aninsulating film 792, an electrode 793, an electrode 794, an insulatingfilm 795, an electrode 796, and an insulating film 797. Changes in themutual capacitance in the electrodes 793 and 794 can be detected when anobject such as a finger or a stylus approaches, for example.

A portion in which the electrode 793 intersects with the electrode 794is illustrated in the upper portion of the transistor 750 illustrated inFIG. 19 and FIG. 20. The electrode 796 is electrically connected to thetwo electrodes 793 between which the electrode 794 is sandwiched throughopenings provided in the insulating film 795. Note that a structure inwhich a region where the electrode 796 is provided is provided in thepixel portion 702 is illustrated in FIG. 19 and FIG. 20 as an example;however, one embodiment of the present invention is not limited thereto.For example, the region where the electrode 796 is provided may beprovided in the source driver circuit portion 704.

The electrode 793 and the electrode 794 are provided in a regionoverlapping with the light-blocking film 738. As illustrated in FIG. 19,it is preferable that the electrode 793 not overlap with thelight-emitting element 782. As illustrated in FIG. 20, it is preferablethat the electrode 793 not overlap with the liquid crystal element 775.In other words, the electrode 793 has an opening in a region overlappingwith the light-emitting element 782 and the liquid crystal element 775.That is, the electrode 793 has a mesh shape. With such a structure, theelectrode 793 does not block light emitted from the light-emittingelement 782, or alternatively the electrode 793 does not block lighttransmitted through the liquid crystal element 775. Thus, sinceluminance is hardly reduced even when the touch panel 791 is provided, adisplay device with high visibility and low power consumption can beachieved. Note that the electrode 794 can have a structure similar tothat of the electrode 793.

Since the electrode 793 and the electrode 794 do not overlap with thelight-emitting element 782, a metal material having low transmittancewith respect to visible light can be used for the electrode 793 and theelectrode 794. Furthermore, since the electrode 793 and the electrode794 do not overlap with the liquid crystal element 775, a metal materialhaving low transmittance with respect to visible light can be used forthe electrode 793 and the electrode 794.

Thus, as compared with the case of using an oxide material whosetransmittance with respect to visible light is high, resistance of theelectrodes 793 and 794 can be reduced, whereby sensitivity of the sensorof the touch panel can be increased.

A conductive nanowire may be used for the electrodes 793, 794, and 796,for example. The nanowire may have a mean diameter of greater than orequal to 1 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm, further preferablygreater than or equal to 5 nm and less than or equal to 25 nm. As thenanowire, a carbon nanotube or a metal nanowire such as an Ag nanowire,a Cu nanowire, or an Al nanowire may be used. For example, in the casewhere an Ag nanowire is used for any one of or all of the electrodes793, 794, and 796, the transmittance with respect to visible light canbe greater than or equal to 89% and sheet resistance can be greater thanor equal to 40 Ω/sq. and less than or equal to 100 Ω/sq.

Although the structure of the in-cell touch panel is illustrated in FIG.19 and FIG. 20, one embodiment of the present invention is not limitedthereto. For example, a touch panel formed over the display device 700,what is called an on-cell touch panel, or a touch panel attached to thedisplay device 700, what is called an out-cell touch panel may be used.

In this manner, the display device of one embodiment of the presentinvention can be combined with various types of touch panels.

At least part of this embodiment can be implemented in combination withany of the other embodiments and example described in this specificationas appropriate.

Embodiment 4

In this embodiment, a semiconductor device that is one embodiment of thepresent invention will be described with reference to FIGS. 21A and 21Band FIG. 22.

<4-1. Structure Example of Semiconductor Device>

FIG. 21A is a top view of a semiconductor device 190 of one embodimentof the present invention. FIG. 21B is a cross-sectional view taken alonga dashed dotted line A1-A2 in FIG. 21A. Note that cross sections in achannel length (L) direction of a transistor Tr1 and in a channel length(L) direction of a transistor Tr2 are included in FIG. 21B. Note thatFIG. 22 is a cross-sectional view taken along a dashed dotted line B1-B2in FIG. 21A. A cross section in a channel width (W) direction of thetransistor Tr1 is included in FIG. 22.

Note that some components (e.g., an insulating film serving as a gateinsulating film) of the semiconductor device 190 and some referencenumerals of components are not illustrated in FIG. 21A to avoidcomplexity. Note that some components and some reference numerals ofcomponents are not illustrated as in FIG. 21A in some cases in drawingsof semiconductor devices described below.

The semiconductor device 190 illustrated in FIGS. 21A and 21B includesthe transistor Tr1 and the transistor Tr2 which overlaps at least partlywith the transistor Tr1. Note that the transistor Tr1 and the transistorTr2 are bottom-gate transistors.

When the transistor Tr1 overlaps at least partly with the transistorTr2, for example, the transistor area can be reduced.

The transistor Tr1 includes a conductive film 104 over a substrate 102,an insulating film 106 over the substrate 102 and the conductive film104, an oxide semiconductor film 108 over the insulating film 106, aconductive film 112 a over the oxide semiconductor film 108, aconductive film 112 b over the oxide semiconductor film 108, aninsulating film 114 over the oxide semiconductor film 108, theconductive film 112 a, and the conductive film 112 b, an insulating film116 over the insulating film 114, and a conductive film 122 c over theinsulating film 116.

The transistor Tr2 includes the conductive film 112 b, the insulatingfilm 114 over the conductive film 112 b, the insulating film 116 overthe insulating film 114, an oxide semiconductor film 128 over theinsulating film 116, a conductive film 122 a over the oxidesemiconductor film 128, a conductive film 122 b over the oxidesemiconductor film 128, an insulating film 124 over the oxidesemiconductor film 128, the conductive film 122 a, and the conductivefilm 122 b, an insulating film 126 over the insulating film 124, and aconductive film 130 over the insulating film 126. Note that theconductive film 130 is connected to the conductive film 122 a through anopening 182 provided in the insulating films 124 and 126.

As illustrated in FIGS. 21A and 21B, the oxide semiconductor film 108and the oxide semiconductor film 128 partly overlap with each other.Note that it is favorable that, as illustrated in FIGS. 21A and 21B, achannel region formed in the oxide semiconductor film 108 of thetransistor Tr1 does not overlap with a channel region formed in theoxide semiconductor film 128 of the transistor Tr2.

If the channel region of the transistor Tr1 overlaps with the channelregion of the transistor Tr2, one of the transistors which is activemight adversely affect the other. In order to avoid the adverse effect,a structure in which the distance between the transistor Tr1 and thetransistor Tr2 is increased, a structure in which a conductive film isprovided between the transistor Tr1 and the transistor Tr2, or the likecan be used. However, the thickness of the semiconductor device isincreased when the former structure is used. Thus, for example, when thesemiconductor device 190 is formed over a flexible substrate or thelike, a problem might arise in the bendability and the like. When thelatter structure is used, there is a problem in that a step of formingthe conductive film is needed and the thickness of the semiconductordevice is increased.

In the semiconductor device 190 of one embodiment of the presentinvention, however, the transistor Tr1 overlaps with the transistor Tr2and their channel regions do not overlap with each other. In addition,since parts of their oxide semiconductor films where the channel regionsare formed overlap with each other, the transistor area can be favorablyreduced.

In addition, the oxide semiconductor film 108 and the oxidesemiconductor film 128 each include In, M (M is Al, Ga, or Y), and Zn.Each of the oxide semiconductor film 108 and the oxide semiconductorfilm 128 preferably includes a region where the atomic proportion of Inis higher than the atomic proportion of M, for example. Note that thesemiconductor device of one embodiment of the present invention is notlimited thereto: each of them may include a region where the atomicproportion of In is lower than the atomic proportion of M or may includea region where the atomic proportion of In is equal to the atomicproportion of M.

It is preferable that the compositions of the oxide semiconductor film108 and the oxide semiconductor film 128 be the same or substantiallythe same. When the compositions of the oxide semiconductor film 108 andthe oxide semiconductor film 128 are the same, the manufacturing costcan be reduced. Note that the semiconductor device of one embodiment ofthe present invention is not limited thereto: the compositions of theoxide semiconductor film 108 and the oxide semiconductor film 128 may bedifferent from each other.

The semiconductor device 190 shown in FIGS. 21A and 21B can be favorablyused for a pixel circuit of a display device. The layout shown in FIGS.21A and 21B can increase the pixel density of the display device. Forexample, even when the pixel density of a display device exceeds 1000ppi (pixel per inch) or 2000 ppi, the aperture ratio of pixels can beincreased owing to the structure shown in FIGS. 21A and 21B. Note thatppi is a unit for describing the number of pixels per inch.

When the semiconductor device 190 shown in FIGS. 21A and 21B is used fora pixel of a display device, the channel length (L) and the channelwidth (W) of a transistor, the line widths of a wiring and an electrodeconnected to the transistor, and the like can be relatively large. Theline width and the like can be larger when the transistor Tr1 and thetransistor Tr2 overlap with each other at least partly as shown in FIGS.21A and 21B than those when the transistor Tr1 and the transistor Tr2are provided on the same plane, for example; thus, variations inprocessing size can be reduced.

In addition, one or both of a conductive film and an insulating film canbe shared by the transistor Tr1 and the transistor Tr2; thus, the numberof masks or steps can be reduced.

In the transistor Tr1, for example, the conductive film 104 serves asthe first gate electrode, the conductive film 112 a serves as the sourceelectrode, the conductive film 112 b serves as the drain electrode, andthe conductive film 122 c serves as the second gate electrode. Inaddition, in the transistor Tr1, the insulating film 106 serves as afirst gate insulating film and the insulating films 114 and 116 serve assecond gate insulating films. In the transistor Tr2, the conductive film112 b serves as the first gate electrode, the conductive film 122 aserves as the source electrode, the conductive film 122 b serves as thedrain electrode, and the conductive film 130 serves as the second gateelectrode. In addition, in the transistor Tr2, the insulating films 114and 116 serve as first gate insulating films and the insulating films124 and 126 serve as second gate insulating films.

Note that in this specification and the like, the insulating film 106may be referred to as a first insulating film, the insulating films 114and 116 may be collectively referred to as a second insulating film, andthe insulating films 124 and 126 may be collectively referred to as athird insulating film.

An insulating film 134 is provided over the conductive film 130. Aninsulating film 136 is provided over the insulating film 134. An opening184 is provided in the insulating films 134 and 136 so as to reach theconductive film 130. In addition, a conductive film 138 is provided overthe insulating film 136. Note that the conductive film 138 is connectedto the conductive film 130 in the opening 184.

In addition, an insulating film 140, an EL layer 142, and a conductivefilm 144 are provided over the conductive film 138. The insulating film140 covers part of a side end portion of the conductive film 138 andprevents a short circuit of the conductive films 138 between adjacentpixels. The EL layer 142 emits light. The light-emitting element 160 iscomposed of the conductive film 138, the EL layer 142, and theconductive film 144. The conductive film 138 serves as one electrode ofthe light-emitting element 160. The conductive film 144 serves as theother electrode of the light-emitting element 160.

As described above, in the semiconductor device of one embodiment of thepresent invention, a plurality of transistors are stacked to be reducedin the transistor area. In addition, since one or both of an insulatingfilm and a conductive film are shared by the plurality of transistors,the number of masks or steps can be reduced.

As shown in FIGS. 21A and 21B, each of the transistor Tr1 and thetransistor Tr2 includes two gate electrodes.

Here, the effect of two gate electrodes will be described with referenceto FIGS. 21A and 21B and FIG. 22.

As shown in FIG. 22, the conductive film 122 c serving as the secondgate electrode is electrically connected to the conductive film 104serving as the first gate electrode in an opening 181. Accordingly, theconductive film 104 and the conductive film 122 c are supplied with thesame potential. In addition, as shown in FIG. 22, the oxidesemiconductor film 108 faces the conductive film 104 and the conductivefilm 122 c and is sandwiched between the conductive films serving as thetwo gate electrodes. The length in the channel width direction of eachof the conductive film 104 and the conductive film 122 c is greater thanthe length in the channel width direction of the oxide semiconductorfilm 108. The entire oxide semiconductor film 108 overlaps with theconductive film 104 and the conductive film 122 c with the insulatingfilms 106, 114, and 116 provided therebetween.

In other words, the conductive film 104 and the conductive film 122 care connected in the opening 181 which is provided in the insulatingfilms 106, 114, and 116 and each include a region located outward fromthe side end portion of the oxide semiconductor film 108. With such astructure, the oxide semiconductor film 108 included in the transistorTr1 can be electrically surrounded by electric fields of the conductivefilm 104 and the conductive film 122 c. That is, the transistor Tr1includes a surrounded-channel (S-channel) structure.

Although the structure in which the first gate electrode is electricallyconnected to the second gate electrode is described above, oneembodiment of the present invention is not limited thereto. For example,as in the transistor Tr2 shown in FIG. 21B, the conductive film 130serving as the second gate electrode may be electrically connected tothe conductive film 122 a serving as the source electrode or the drainelectrode of the transistor Tr2.

<4-2. Components of Semiconductor Device>

Next, components of the semiconductor device of this embodiment will bedescribed in detail. Note that components that are similar to thecomponents in Embodiment 1 are denoted by the same reference numerals,and detailed description thereof is omitted.

[Conductive Film]

The conductive film 122 a, the conductive film 122 b, the conductivefilm 122 c, the conductive film 130, the conductive film 138, and theconductive film 144 can be formed using a material similar to that forthe conductive film 104, the conductive film 112 a, the conductive film112 b, the conductive film 120 a, and the conductive film 120 b.

The conductive film 122 a, the conductive film 122 b, the conductivefilm 122 c, the conductive film 130, the conductive film 138, and theconductive film 144 can each be formed using an oxide conductor (OC)such as an oxide including indium and tin, an oxide including tungstenand indium, an oxide including tungsten, indium, and zinc, an oxideincluding titanium and indium, an oxide including titanium, indium, andtin, an oxide including indium and zinc, an oxide including silicon,indium, and tin, an oxide including indium, gallium, and zinc.

The above-listed oxide conductors (OC) are particularly favorable as theconductive film 130.

[Insulating Film]

The insulating film 124, the insulating film 126, and the insulatingfilm 134 can be formed using a material similar to that for theinsulating film 106, the insulating film 114, and the insulating film116.

Note that an insulating film that is in contact with one or both of theoxide semiconductor film 108 and the oxide semiconductor film 128 ispreferably an oxide insulating film and preferably includes a regioncontaining oxygen in excess of that in the stoichiometric composition(the oxygen-excess region). In other words, the oxide insulating filmincluding the oxygen-excess region is an insulating film capable ofreleasing oxygen.

The oxygen-excess region of the oxide insulating film can be formed byany of the following methods, for example: an insulating film is formedin an oxygen atmosphere; an insulating film is formed, and then issubjected to heat treatment in an oxygen atmosphere; or an insulatingfilm is formed, and then oxygen is added to the insulating film. Plasmatreatment is preferable for adding oxygen into the formed insulatingfilm.

The insulating film serving as the gate insulating film of each of thetransistor Tr1 and the transistor Tr2 may be formed using siliconnitride. When silicon nitride is used for the insulating film serving asthe gate insulating film, the following effects are obtained. Siliconnitride has a higher dielectric constant than silicon oxide and needs alarger thickness to obtain capacitance equivalent to that of siliconoxide. Thus, the thickness of the insulating film can be increased. Thismakes it possible to prevent a decrease in withstand voltage of thetransistor Tr1 and the transistor Tr2 and to increase the withstandvoltage, thereby preventing electrostatic discharge of the transistorTr1 and the transistor Tr2.

The insulating films 114, 116, 124, and 126 have a function of supplyingoxygen to the oxide semiconductor film 108 and/or the oxidesemiconductor film 128. That is, the insulating films 114, 116, 124, and126 contain oxygen. The insulating films 114 and 124 are insulatingfilms which allow passage of oxygen. Note that the insulating film 114also functions as a film for relieving damage to the oxide semiconductorfilm 108 at the time of forming the insulating film 116 in a later step.The insulating film 124 also functions as a film for relieving damage tothe oxide semiconductor film 128 at the time of forming the insulatingfilm 126 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the insulating films 114 and 124.

In addition, it is preferable that the number of defects in theinsulating films 114 and 124 be small, and typically, the spin densityof a signal that appears at g=2.001 due to a dangling bond of silicon belower than or equal to 3×10¹⁷ spins/cm³ when measured by ESRmeasurement. This is because if the density of defects in each of theinsulating films 114 and 124 is high, oxygen is bonded to the defectsand the amount of oxygen that passes through the insulating films 114and 124 is decreased.

The insulating films 114 and 124 can each be formed using an oxideinsulating film having a low density of states due to nitrogen oxide.Note that the density of states due to nitrogen oxide can be formedbetween the energy of the valence band maximum (E_(v) _(_) _(os)) andthe energy of the conduction band minimum (E_(c) _(_) _(os)) of theoxide semiconductor film. A silicon oxynitride film that releases lessnitrogen oxide, an aluminum oxynitride film that releases less nitrogenoxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide isa film of which the release amount of ammonia is larger than the releaseamount of nitrogen oxide in thermal desorption spectroscopy (TDS); therelease amount of ammonia is typically greater than or equal to 1×10¹⁸cm⁻³ and less than or equal to 5×10¹⁹ cm⁻³. Note that the release amountof ammonia is the total amount of ammonia released by heat treatment ata temperature in the range of 50° C. to 650° C. or the range of 50° C.to 550° C. in TDS. The release amount of ammonia is the total releaseamount of ammonia converted into ammonia molecules in TDS.

The insulating film 134 serves as a protective insulating film of eachof the transistor Tr1 and the transistor Tr2.

The insulating film 134 contains one or both of hydrogen and nitrogen.Alternatively, the insulating film 134 contains nitrogen and silicon.The insulating film 134 has a function of blocking oxygen, hydrogen,water, an alkali metal, an alkaline earth metal, or the like. It ispossible to prevent outward diffusion of oxygen from the oxidesemiconductor film 108 and the oxide semiconductor film 128, outwarddiffusion of oxygen included in the insulating films 114, 116, 124, and126, and entry of hydrogen, water, or the like into the oxidesemiconductor films 108 and 128 from the outside by providing theinsulating film 134.

The insulating film 134 can be formed using a nitride insulating film,for example. The nitride insulating film is formed using siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like.

The insulating film 136 and the insulating film 140 each has a functionof covering unevenness and the like caused by the transistor or thelike. Each of the insulating films 136 and 140 has an insulatingproperty and is formed using an inorganic or organic material. Examplesof the inorganic material include a silicon oxide film, a siliconoxynitride film, a silicon nitride oxide film, a silicon nitride film,an aluminum oxide film, and an aluminum nitride film. Examples of theorganic material include photosensitive resin materials such as anacrylic resin and a polyimide resin.

[Oxide Semiconductor Film]

The oxide semiconductor film 128 can be formed using a material similarto that of the oxide semiconductor film 108.

[EL Layer]

The EL layer 142 has a function of emitting light and includes at leasta light-emitting layer. Other than the light-emitting layer, the ELlayer 142 includes functional layers such as a hole-injection layer, ahole-transport layer, an electron-transport layer, and anelectron-injection layer. A low molecular compound or a high molecularcompound can be used for the EL layer 142.

At least part of this embodiment can be implemented in combination withany of the other embodiments and example described in this specificationas appropriate.

Embodiment 5

In this embodiment, an example of a display panel which can be used fora display portion or the like in a display device including thesemiconductor device of one embodiment of the present invention will bedescribed with reference to FIG. 23 and FIG. 24. The display paneldescribed below as an example includes both a reflective liquid crystalelement and a light-emitting element and can display an image in boththe transmissive mode and the reflective mode.

<5-1. Structure Example of Display Panel>

FIG. 23 is a schematic perspective view illustrating a display panel 600of one embodiment of the present invention. In the display panel 600, asubstrate 651 and a substrate 661 are attached to each other. In FIG.23, the substrate 661 is denoted by a broken line.

The display panel 600 includes a display portion 662, a circuit 659, awiring 666, and the like. The substrate 651 is provided with the circuit659, the wiring 666, a conductive film 663 which serves as a pixelelectrode, and the like. In FIG. 23, an IC 673 and an FPC 672 aremounted on the substrate 651. Thus, the structure illustrated in FIG. 23can be referred to as a display module including the display panel 600,the FPC 672, and the IC 673.

As the circuit 659, for example, a circuit functioning as a scan linedriver circuit can be used.

The wiring 666 has a function of supplying a signal or electric power tothe display portion or the circuit 659. The signal or electric power isinput to the wiring 666 from the outside through the FPC 672 or from theIC 673.

FIG. 23 shows an example in which the IC 673 is provided on thesubstrate 651 by a chip on glass (COG) method or the like. As the IC673, an IC functioning as a scan line driver circuit, a signal linedriver circuit, or the like can be used. Note that it is possible thatthe IC 673 is not provided when, for example, the display panel 600includes circuits serving as a scan line driver circuit and a signalline driver circuit and when the circuits serving as a scan line drivercircuit and a signal line driver circuit are provided outside and asignal for driving the display panel 600 is input through the FPC 672.Alternatively, the IC 673 may be mounted on the FPC 672 by a chip onfilm (COF) method or the like.

FIG. 23 also shows an enlarged view of part of the display portion 662.The conductive films 663 included in a plurality of display elements arearranged in a matrix in the display portion 662. The conductive film 663has a function of reflecting visible light and serves as a reflectiveelectrode of a liquid crystal element 640 described later.

As illustrated in FIG. 23, the conductive film 663 has an opening. Alight-emitting element 660 is positioned closer to the substrate 651than the conductive film 663 is. Light is emitted from thelight-emitting element 660 to the substrate 661 side through the openingin the conductive film 663.

<5-2. Cross-Sectional Structure Example>

FIG. 24 shows an example of cross sections of part of a region includingthe FPC 672, part of a region including the circuit 659, and part of aregion including the display portion 662 of the display panelillustrated in FIG. 23.

The display panel includes an insulating film 620 between the substrates651 and 661. The display panel also includes the light-emitting element660, a transistor 601, a transistor 605, a transistor 606, a coloringlayer 634, and the like between the substrate 651 and the insulatingfilm 620. Furthermore, the display panel includes the liquid crystalelement 640, a coloring layer 631, and the like between the insulatingfilm 620 and the substrate 661. The substrate 661 and the insulatingfilm 620 are bonded with an adhesive layer 641. The substrate 651 andthe insulating film 620 are bonded with an adhesive layer 642.

The transistor 606 is electrically connected to the liquid crystalelement 640 and the transistor 605 is electrically connected to thelight-emitting element 660. Since the transistors 605 and 606 are formedon a surface of the insulating film 620 which is on the substrate 651side, the transistors 605 and 606 can be formed through the sameprocess.

The substrate 661 is provided with the coloring layer 631, alight-blocking film 632, an insulating film 621, a conductive film 613serving as a common electrode of the liquid crystal element 640, analignment film 633 b, an insulating film 617, and the like. Theinsulating film 617 serves as a spacer for holding a cell gap of theliquid crystal element 640.

Insulating layers such as an insulating film 681, an insulating film682, an insulating film 683, an insulating film 684, and an insulatingfilm 685 are provided on the substrate 651 side of the insulating film620. Part of the insulating film 681 functions as a gate insulatinglayer of each transistor. The insulating films 682, 683, and 684 areprovided to cover each transistor. The insulating film 685 is providedto cover the insulating film 684. The insulating films 684 and 685 eachfunction as a planarization layer. Note that an example where the threeinsulating layers, the insulating films 682, 683, and 684, are providedto cover the transistors and the like is described here; however, oneembodiment of the present invention is not limited to this example, andfour or more insulating layers, a single insulating layer, or twoinsulating layers may be provided. The insulating film 684 functioningas a planarization layer is not necessarily provided when not needed.

The transistors 601, 605, and 606 each include a conductive film 654part of which functions as a gate, a conductive film 652 part of whichfunctions as a source or a drain, and a semiconductor film 653. Here, aplurality of layers obtained by processing the same conductive film areshown with the same hatching pattern.

The liquid crystal element 640 is a reflective liquid crystal element.The liquid crystal element 640 has a stacked structure of a conductivefilm 635, a liquid crystal layer 612, and the conductive film 613. Inaddition, the conductive film 663 which reflects visible light isprovided in contact with the surface of the conductive film 635 thatfaces the substrate 651. The conductive film 663 includes an opening655. The conductive films 635 and 613 contain a material transmittingvisible light. In addition, an alignment film 633 a is provided betweenthe liquid crystal layer 612 and the conductive film 635 and thealignment film 633 b is provided between the liquid crystal layer 612and the conductive film 613. A polarizing plate 656 is provided on anouter surface of the substrate 661.

In the liquid crystal element 640, the conductive film 663 has afunction of reflecting visible light and the conductive film 613 has afunction of transmitting visible light. Light entering from thesubstrate 661 side is polarized by the polarizing plate 656, passesthrough the conductive film 613 and the liquid crystal layer 612, and isreflected by the conductive film 663. Then, the light passes through theliquid crystal layer 612 and the conductive film 613 again and reachesthe polarizing plate 656. In this case, alignment of the liquid crystalis controlled with a voltage that is applied between the conductive film613 and the conductive film 663, and thus optical modulation of lightcan be controlled. That is, the intensity of light emitted through thepolarizing plate 656 can be controlled. Light other than that in aparticular wavelength region is absorbed by the coloring layer 631, andthus, emitted light is red light, for example.

The light-emitting element 660 is a bottom-emission light-emittingelement. The light-emitting element 660 has a structure in which aconductive film 643, an EL layer 644, and a conductive film 645 b arestacked in this order from the insulating film 620 side. In addition, aconductive film 645 a is provided to cover the conductive film 645 b.The conductive film 645 b contains a material reflecting visible light,and the conductive films 643 and 645 a contain a material transmittingvisible light. Light is emitted from the light-emitting element 660 tothe substrate 661 side through the coloring layer 634, the insulatingfilm 620, the opening 655, the conductive film 613, and the like.

Here, as illustrated in FIG. 24, the conductive film 635 transmittingvisible light is preferably provided for the opening 655. Accordingly,the liquid crystal layer 612 is aligned in a region overlapping with theopening 655 as well as in the other regions, in which case an alignmentdefect of the liquid crystal is prevented from being generated in theboundary portion of these regions and undesired light leakage can besuppressed.

As the polarizing plate 656 provided on an outer surface of thesubstrate 661, a linear polarizing plate or a circularly polarizingplate can be used. An example of a circularly polarizing plate is astack including a linear polarizing plate and a quarter-wave retardationplate. Such a structure can reduce reflection of external light. Thecell gap, alignment, drive voltage, and the like of the liquid crystalelement used as the liquid crystal element 640 are controlled dependingon the kind of the polarizing plate so that desirable contrast isachieved.

In addition, an insulating film 647 is provided in contact with theinsulating film 646 covering an end portion of the conductive film 643.The insulating film 647 has a function as a spacer for preventing theinsulating film 620 and the substrate 651 from getting closer more thannecessary. In the case where the EL layer 644 or the conductive film 645a is formed using a blocking mask (metal mask), the insulating film 647may have a function as a spacer for preventing the blocking mask frombeing in contact with a surface on which the EL layer 644 or theconductive film 645 a is formed. Note that the insulating film 647 isnot necessarily provided when not needed.

One of a source and a drain of the transistor 605 is electricallyconnected to the conductive film 643 of the light-emitting element 660through a conductive film 648.

One of a source and a drain of the transistor 606 is electricallyconnected to the conductive film 663 through a connection portion 607.The conductive films 663 and 635 are in contact with and electricallyconnected to each other. Here, in the connection portion 607, theconductive layers provided on both surfaces of the insulating film 620are connected to each other through an opening in the insulating film620.

A connection portion 604 is provided in a region where the substrates651 and 661 do not overlap with each other. The connection portion 604is electrically connected to the FPC 672 through a connection layer 649.The connection portion 604 has a structure similar to that of theconnection portion 607. On the top surface of the connection portion604, a conductive layer obtained by processing the same conductive filmas the conductive film 635 is exposed. Thus, the connection portion 604and the FPC 672 can be electrically connected to each other through theconnection layer 649.

A connection portion 687 is provided in part of a region where theadhesive layer 641 is provided. In the connection portion 687, theconductive layer obtained by processing the same conductive film as theconductive film 635 is electrically connected to part of the conductivefilm 613 with a connector 686. Accordingly, a signal or a potentialinput from the FPC 672 connected to the substrate 651 side can besupplied to the conductive film 613 formed on the substrate 661 sidethrough the connection portion 687.

As the connector 686, a conductive particle can be used, for example. Asthe conductive particle, a particle of an organic resin, silica, or thelike coated with a metal material can be used. It is preferable to usenickel or gold as the metal material because contact resistance can bereduced. It is also preferable to use a particle coated with layers oftwo or more kinds of metal materials, such as a particle coated withnickel and further with gold. As the connector 686, a material capableof elastic deformation or plastic deformation is preferably used. Asillustrated in FIG. 24, the connector 686 which is the conductiveparticle has a shape that is vertically crushed in some cases. With thecrushed shape, the contact area between the connector 686 and aconductive layer electrically connected to the connector 686 can beincreased, thereby reducing contact resistance and suppressing thegeneration of problems such as disconnection.

The connector 686 is preferably provided so as to be covered with theadhesive layer 641. For example, the connector 686 is dispersed in theadhesive layer 641 before curing of the adhesive layer 641.

FIG. 24 illustrates an example of the circuit 659 in which thetransistor 601 is provided.

The structure in which the semiconductor film 653 where a channel isformed is provided between two gates is used as an example of thetransistors 601 and 605 in FIG. 24. One gate is formed using theconductive film 654 and the other gate is formed using a conductive film623 overlapping with the semiconductor film 653 with the insulating film682 provided therebetween. Such a structure enables control of thresholdvoltages of a transistor. In that case, the two gates may be connectedto each other and supplied with the same signal to operate thetransistor. Such a transistor can have higher field-effect mobility andthus have higher on-state current than other transistors. Consequently,a circuit capable of high-speed operation can be obtained. Furthermore,the area occupied by a circuit portion can be reduced. The use of thetransistor having high on-state current can reduce signal delay inwirings and can reduce display unevenness even in a display panel inwhich the number of wirings is increased because of increase in size orresolution.

Note that the transistor included in the circuit 659 and the transistorincluded in the display portion 662 may have the same structure. Aplurality of transistors included in the circuit 659 may have the samestructure or different structures. A plurality of transistors includedin the display portion 662 may have the same structure or differentstructures.

A material through which impurities such as water and hydrogen do noteasily diffuse is preferably used for at least one of the insulatingfilms 682 and 683 which cover the transistors. That is, the insulatingfilm 682 or the insulating film 683 can function as a barrier film. Sucha structure can effectively suppress diffusion of the impurities intothe transistors from the outside, and a highly reliable display panelcan be achieved.

The insulating film 621 is provided on the substrate 661 side to coverthe coloring layer 631 and the light-blocking film 632. The insulatingfilm 621 may have a function as a planarization layer. The insulatingfilm 621 enables the conductive film 613 to have an almost flat surface,resulting in a uniform alignment state of the liquid crystal layer 612.

An example of the manufacturing method of the display panel 600 isdescribed. For example, the conductive film 635, the conductive film663, and the insulating film 620 are formed in order over a supportsubstrate provided with a separation layer, and the transistor 605, thetransistor 606, the light-emitting element 660, and the like are formed.Then, the substrate 651 and the support substrate are bonded with theadhesive layer 642. After that, separation is performed at the interfacebetween the separation layer and each of the insulating film 620 and theconductive film 635, whereby the support substrate and the separationlayer are removed. Separately, the coloring layer 631, thelight-blocking film 632, the conductive film 613, and the like areformed over the substrate 661 in advance. Then, a liquid crystal isdropped onto the substrate 651 or 661 and the substrates 651 and 661 arebonded with the adhesive layer 641, whereby the display panel 600 can bemanufactured.

A material for the separation layer can be selected such that separationat the interface with the insulating film 620 and the conductive film635 occurs. In particular, it is preferable that a stacked layer of alayer including a high-melting-point metal material, such as tungsten,and a layer including an oxide of the metal material be used as theseparation layer, and a stacked layer of a plurality of layers, such asa silicon nitride layer, a silicon oxynitride layer, and a siliconnitride oxide layer be used as the insulating film 620 over theseparation layer. The use of the high-melting-point metal material forthe separation layer can increase the formation temperature of a layerformed in a later step, which reduces impurity concentration andachieves a highly reliable display panel.

As the conductive film 635, an oxide or a nitride such as a metal oxide,a metal nitride, or an oxide semiconductor whose resistance is reducedis preferably used. In the case of using an oxide semiconductor, amaterial in which at least one of the concentrations of hydrogen, boron,phosphorus, nitrogen, and other impurities and the number of oxygenvacancies is made to be higher than those in a semiconductor layer of atransistor is used for the conductive film 635.

<5-3. Components>

The above components will be described below. Note that descriptions ofstructures having functions similar to those in the above embodimentsare omitted.

[Adhesive Layer]

As the adhesive layer, a variety of curable adhesives such as a reactivecurable adhesive, a thermosetting adhesive, an anaerobic adhesive, and aphotocurable adhesive such as an ultraviolet curable adhesive can beused. Examples of these adhesives include an epoxy resin, an acrylicresin, a silicone resin, a phenol resin, a polyimide resin, an imideresin, a polyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB)resin, and an ethylene vinyl acetate (EVA) resin. In particular, amaterial with low moisture permeability, such as an epoxy resin, ispreferred. Alternatively, a two-component-mixture-type resin may beused. Further alternatively, an adhesive sheet or the like may be used.

Furthermore, the resin may include a drying agent. For example, asubstance that adsorbs moisture by chemical adsorption, such as an oxideof an alkaline earth metal (e.g., calcium oxide or barium oxide), can beused. Alternatively, a substance that adsorbs moisture by physicaladsorption, such as zeolite or silica gel, may be used. The drying agentis preferably included because it can prevent impurities such asmoisture from entering the element, thereby improving the reliability ofthe display panel.

In addition, it is preferable to mix a filler with a high refractiveindex or light-scattering member into the resin, in which case lightextraction efficiency can be enhanced. For example, titanium oxide,barium oxide, zeolite, zirconium, or the like can be used.

[Connection Layer]

As the connection layer, an anisotropic conductive film (ACF), ananisotropic conductive paste (ACP), or the like can be used.

[Coloring Layer]

Examples of a material that can be used for the coloring layers includea metal material, a resin material, and a resin material containing apigment or dye.

[Light-Blocking Layer]

Examples of a material that can be used for the light-blocking layerinclude carbon black, titanium black, a metal, a metal oxide, and acomposite oxide containing a solid solution of a plurality of metaloxides. The light-blocking layer may be a film containing a resinmaterial or a thin film of an inorganic material such as a metal.Stacked films containing the material of the coloring layer can also beused for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material of a coloring layer whichtransmits light of a certain color and a film containing a material of acoloring layer which transmits light of another color can be employed.It is preferable that the coloring layer and the light-blocking layer beformed using the same material because the same manufacturing apparatuscan be used and the process can be simplified.

The above is the description of the components.

<5-4. Manufacturing Method Example>

A manufacturing method example of a display panel using a flexiblesubstrate will be described.

Here, layers including a display element, a circuit, a wiring, anelectrode, optical members such as a coloring layer and a light-blockinglayer, an insulating layer, and the like, are collectively referred toas an element layer. The element layer includes, for example, a displayelement, and may additionally include a wiring electrically connected tothe display element or an element such as a transistor used in a pixelor a circuit.

In addition, here, a flexible member which supports the element layer ata stage at which the display element is completed (the manufacturingprocess is finished) is referred to as a substrate. For example, asubstrate includes an extremely thin film with a thickness greater thanor equal to 10 nm and less than or equal to 300 μm and the like.

As a method for forming an element layer over a flexible substrateprovided with an insulating surface, typically, there are two methodsshown below. One of them is to directly form an element layer over thesubstrate. The other method is to form an element layer over a supportsubstrate that is different from the substrate and then to separate theelement layer from the support substrate to be transferred to thesubstrate. Although not described in detail here, in addition to theabove two methods, there is a method in which an element layer is formedover a substrate which does not have flexibility and the substrate isthinned by polishing or the like to have flexibility.

In the case where a material of the substrate can withstand heatingtemperature in a process for forming the element layer, it is preferablethat the element layer be formed directly over the substrate, in whichcase a manufacturing process can be simplified. At this time, theelement layer is preferably formed in a state where the substrate isfixed to the support substrate, in which case transfer thereof in anapparatus and between apparatuses can be easy.

In the case of employing the method in which the element layer is formedover the support substrate and then transferred to the substrate, first,a separation layer and an insulating layer are stacked over the supportsubstrate, and then the element layer is formed over the insulatinglayer. Next, the element layer is separated from the support substrateand then transferred to the substrate. At this time, selected is amaterial with which separation at an interface between the supportsubstrate and the separation layer, at an interface between theseparation layer and the insulating layer, or in the separation layeroccurs. With the method, it is preferable that a material having highheat resistance be used for the support substrate or the separationlayer, in which case the upper limit of the temperature applied when theelement layer is formed can be increased, and an element layer includinga more highly reliable element can be formed.

It is preferable that a stack of a layer containing a high-melting-pointmetal material, such as tungsten, and a layer containing an oxide of themetal material be used as the separation layer, and a stack of aplurality of layers, such as a silicon oxide layer, a silicon nitridelayer, a silicon oxynitride layer, and a silicon nitride oxide layer beused as the insulating layer over the separation layer, for example.

As the method for separating the support substrate from the elementlayer, applying mechanical force, etching the separation layer, andmaking a liquid permeate the separation interface are given as examples.Alternatively, separation may be performed by heating or cooling twolayers of the separation interface by utilizing a difference in thermalexpansion coefficient.

The separation layer is not necessarily provided in the case where theseparation can be performed at an interface between the supportsubstrate and the insulating layer.

Glass and an organic resin such as polyimide can be used as the supportsubstrate and the insulating layer, respectively, for example. In thatcase, a separation trigger may be formed by, for example, locallyheating part of the organic resin with laser light or the like, or byphysically cutting part of or making a hole through the organic resinwith a sharp tool, and separation may be performed at an interfacebetween the glass and the organic resin. As the above-described organicresin, a photosensitive material is favorably used because an opening orthe like can be easily formed. The above-described laser lightpreferably has a wavelength region, for example, from visible light toultraviolet light. For example, light having a wavelength of greaterthan or equal to 200 nm and less than or equal to 400 nm, preferablygreater than or equal to 250 nm and less than or equal to 350 nm can beused. In particular, an excimer laser having a wavelength of 308 nm ispreferably used because the productivity is increased. Alternatively, asolid-state UV laser (also referred to as a semiconductor UV laser),such as a UV laser having a wavelength of 355 nm which is the thirdharmonic of an Nd:YAG laser, may be used.

Alternatively, a heat generation layer may be provided between thesupport substrate and the insulating layer formed of an organic resin,and separation may be performed at an interface between the heatgeneration layer and the insulating layer by heating the heat generationlayer. For the heat generation layer, any of a variety of materials suchas a material which generates heat by feeding current, a material whichgenerates heat by absorbing light, and a material which generates heatby applying a magnetic field can be used. For example, for the heatgeneration layer, a material selected from a semiconductor, a metal, andan insulator can be used.

In the above-described methods, the insulating layer formed of anorganic resin can be used as a substrate after the separation.

The above is the description of a manufacturing method of a flexibledisplay panel.

At least part of this embodiment can be implemented in combination withany of the other embodiments and example described in this specificationas appropriate.

Embodiment 6

In this embodiment, a display device including a semiconductor device ofone embodiment of the present invention will be described with referenceto FIG. 25A to 25C.

<6. Circuit Configuration of Display Device>

A display device illustrated in FIG. 25A includes a region includingpixels of display elements (hereinafter referred to as a pixel portion502), a circuit portion that is provided outside the pixel portion 502and includes a circuit for driving the pixels (hereinafter, the circuitportion is referred to as a driver circuit portion 504), circuits havinga function of protecting elements (hereinafter, the circuits arereferred to as protection circuits 506), and a terminal portion 507.Note that the protection circuits 506 are not necessarily provided.

Part or the whole of the driver circuit portion 504 is preferably formedover a substrate over which the pixel portion 502 is formed. Thus, thenumber of components and the number of terminals can be reduced. Whenpart or the whole of the driver circuit portion 504 is not formed overthe substrate over which the pixel portion 502 is formed, the part orthe whole of the driver circuit portion 504 can be mounted by COG ortape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X (X is a natural number of 2 or more) rowsand Y (Y is a natural number of 2 or more) columns (hereinafter, thecircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter referred to as scan lines GL_1 to GL_X). Note that aplurality of gate drivers 504 a may be provided to control the scanlines GL_1 to GL_X separately. Alternatively, the gate driver 504 a hasa function of supplying an initialization signal. Without being limitedthereto, another signal can be supplied from the gate driver 504 a.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (image signal) from which a datasignal is generated, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 fromthe image signal. In addition, the source driver 504 b has a function ofcontrolling output of a data signal in response to a pulse signalproduced by input of a start pulse signal, a clock signal, or the like.Furthermore, the source driver 504 b has a function of controlling thepotentials of wirings supplied with data signals (hereinafter referredto as data lines DL_1 to DL_Y). Alternatively, the source driver 504 bhas a function of supplying an initialization signal. Without beinglimited thereto, another signal can be supplied from the source driver504 b.

The source driver 504 b includes a plurality of analog switches, forexample. The source driver 504 b can output, as data signals,time-divided image signals obtained by sequentially turning on theplurality of analog switches. The source driver 504 b may include ashift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of X orless, and n is a natural number of Y or less), a pulse signal is inputfrom the gate driver 504 a through the scan line GL_m, and a data signalis input from the source driver 504 b through the data line DL_n inaccordance with the potential of the scan line GL_m.

The protection circuit 506 in FIG. 25A is connected to, for example, thescan line GL between the gate driver 504 a and the pixel circuit 501.Alternatively, the protection circuit 506 is connected to the data lineDL between the source driver 504 b and the pixel circuit 501.Alternatively, the protection circuit 506 can be connected to a wiringbetween the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 refers to a portion having terminals forinputting power, control signals, and image signals from externalcircuits to the display device.

The protection circuit 506 electrically connects a wiring connected tothe protection circuit to another wiring when a potential out of acertain range is supplied to the wiring connected to the protectioncircuit.

As illustrated in FIG. 25A, the protection circuits 506 provided for thepixel portion 502 and the driver circuit portion 504 can improve theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like. Note that the configurationof the protection circuits 506 is not limited thereto; for example, theprotection circuit 506 can be connected to the gate driver 504 a or thesource driver 504 b. Alternatively, the protection circuit 506 can beconnected to the terminal portion 507.

One embodiment of the present invention is not limited to the example inFIG. 25A, in which the driver circuit portion 504 includes the gatedriver 504 a and the source driver 504 b. For example, only the gatedriver 504 a may be formed, and a separately prepared substrate overwhich a source driver circuit is formed (e.g., a driver circuit boardformed using a single crystal semiconductor film or a polycrystallinesemiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 25A can have theconfiguration illustrated in FIG. 25B, for example.

The pixel circuit 501 in FIG. 25B includes a liquid crystal element 570,a transistor 550, and a capacitor 560. As the transistor 550, thetransistor described in the above embodiment can be used.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set as appropriate in accordance with the specificationsof the pixel circuit 501. The alignment state of the liquid crystalelement 570 depends on data written thereto. A common potential may besupplied to the one of the pair of electrodes of the liquid crystalelement 570 included in each of the plurality of pixel circuits 501. Thepotential supplied to the one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 may differ between rows.

Examples of a method for driving the display device including the liquidcrystal element 570 include a TN mode, an STN mode, a VA mode, anaxially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, an MVAmode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFSmode, and a transverse bend alignment (TBA) mode. Other examples of themethod for driving the display device include an electrically controlledbirefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC)mode, a polymer network liquid crystal (PNLC) mode, and a guest-hostmode. Without being limited thereto, various liquid crystal elements anddriving methods can be used.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other of thesource electrode and the drain electrode of the transistor 550 iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 isconfigured to be turned on or off to control whether a data signal iswritten.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring through which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other of the pair ofelectrodes of the capacitor 560 is electrically connected to the otherof the pair of electrodes of the liquid crystal element 570. Thepotential of the potential supply line VL is set as appropriate inaccordance with the specifications of the pixel circuit 501. Thecapacitor 560 functions as a storage capacitor for storing written data.

In the display device including the pixel circuits 501 in FIG. 25B, forexample, the gate driver 504 a in FIG. 25A sequentially selects thepixel circuits 501 row by row to turn on the transistors 550, and datasignals are written.

When the transistor 550 is turned off, the pixel circuit 501 to whichthe data has been written is brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 25Acan have the configuration illustrated in FIG. 25C, for example.

The pixel circuit 501 in FIG. 25C includes transistors 552 and 554, acapacitor 562, and a light-emitting element 572. The transistordescribed in the above embodiment can be used as the transistor 552and/or the transistor 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring through which a data signal issupplied (hereinafter referred to as a data line DL_n). A gate electrodeof the transistor 552 is electrically connected to a wiring throughwhich a gate signal is supplied (hereinafter referred to as a scan lineGL_m).

The transistor 552 is configured to be turned on or off to controlwhether a data signal is written.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring through which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other of the pairof electrodes of the capacitor 562 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. A gateelectrode of the transistor 554 is electrically connected to the otherof the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other ofthe anode and the cathode of the light-emitting element 572 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) can be used, for example.Note that the light-emitting element 572 is not limited thereto and maybe an inorganic EL element including an inorganic material.

A high power supply potential V_(DD) is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential V_(SS) is supplied to the other of the potential supplyline VL_a and the potential supply line VL_b.

In the display device including the pixel circuits 501 in FIG. 25C, thegate driver 504 a in FIG. 25A sequentially selects the pixel circuits501 row by row to turn on the transistors 552, and data signals arewritten.

When the transistor 552 is turned off, the pixel circuit 501 to whichthe data has been written is brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

At least part of this embodiment can be implemented in combination withany of the other embodiments and example described in this specificationas appropriate.

Embodiment 7

In this embodiment, a display module and electronic devices, each ofwhich includes a semiconductor device of one embodiment of the presentinvention, will be described with reference to FIG. 26, FIGS. 27A to27E, and FIGS. 28A to 28G.

<7-1. Display Module>

In a display module 7000 illustrated in FIG. 26, a touch panel 7004connected to an FPC 7003, a display panel 7006 connected to an FPC 7005,a backlight 7007, a frame 7009, a printed-circuit board 7010, and abattery 7011 are provided between an upper cover 7001 and a lower cover7002.

The semiconductor device of one embodiment of the present invention canbe used for the display panel 7006, for example.

The shapes and sizes of the upper cover 7001 and the lower cover 7002can be changed as appropriate in accordance with the sizes of the touchpanel 7004 and the display panel 7006.

The touch panel 7004 can be a resistive touch panel or a capacitivetouch panel and overlap with the display panel 7006. Alternatively, acounter substrate (sealing substrate) of the display panel 7006 can havea touch panel function. Alternatively, a photosensor may be provided ineach pixel of the display panel 7006 to form an optical touch panel.

The backlight 7007 includes a light source 7008. One embodiment of thepresent invention is not limited to the structure in FIG. 26, in whichthe light source 7008 is provided over the backlight 7007. For example,a structure in which the light source 7008 is provided at an end portionof the backlight 7007 and a light diffusion plate is further providedmay be employed. Note that the backlight 7007 need not be provided inthe case where a self-luminous light-emitting element such as an organicEL element is used or in the case where a reflective panel or the likeis employed.

The frame 7009 protects the display panel 7006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed-circuit board 7010. The frame 7009 may alsofunction as a radiator plate.

The printed-circuit board 7010 includes a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or the separate battery7011 may be used. The battery 7011 can be omitted in the case where acommercial power source is used.

The display module 7000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<7-2. Electronic Device 1>

Next, FIGS. 27A to 27E illustrate examples of electronic devices.

FIG. 27A is an external view of a camera 8000 to which a finder 8100 isattached.

The camera 8000 includes a housing 8001, a display portion 8002, anoperation button 8003, a shutter button 8004, and the like. Furthermore,an attachable lens 8006 is attached to the camera 8000.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in thehousing 8001.

Images can be taken with the camera 8000 at the press of the shutterbutton 8004. In addition, images can be taken at the touch of thedisplay portion 8002 that serves as a touch panel.

The housing 8001 of the camera 8000 includes a mount including anelectrode, so that the finder 8100, a stroboscope, or the like can beconnected to the housing 8001.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a mount for engagement with the mount of thecamera 8000 so that the finder 8100 can be connected to the camera 8000.The mount includes an electrode, and an image or the like received fromthe camera 8000 through the electrode can be displayed on the displayportion 8102.

The button 8103 serves as a power button. The on/off state of thedisplay portion 8102 can be switched with the button 8103.

A display device of one embodiment of the present invention can be usedin the display portion 8002 of the camera 8000 and the display portion8102 of the finder 8100.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIG. 27A, the housing 8001 of the camera 8000 mayinclude a finder having a display device.

FIG. 27B is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. The movement of the eyeball and the eyelid of auser is captured by a camera in the main body 8203 and then coordinatesof the points the user looks at are calculated using the captured datato utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as tobe in contact with the user. The main body 8203 may be configured tosense current flowing through the electrodes with the movement of theuser's eyeball to recognize the direction of his or her eyes. The mainbody 8203 may be configured to sense current flowing through theelectrodes to monitor the user's pulse. The mounting portion 8201 mayinclude sensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The display device of one embodiment of the present invention can beused in the display portion 8204.

FIGS. 27C to 27E are external views of a head-mounted display 8300. Thehead-mounted display 8300 includes a housing 8301, a display portion8302, an object for fixing, such as a band, 8304, and a pair of lenses8305.

A user can see display on the display portion 8302 through the lenses8305. It is favorable that the display portion 8302 is curved. When thedisplay portion 8302 is curved, a user can feel high realistic sensationof images. Although the structure described in this embodiment as anexample has one display portion 8302, the number of the display portions8302 provided is not limited to one. For example, two display portions8302 may be provided, in which case one display portion is provided forone corresponding user's eye, so that three-dimensional display usingparallax or the like is possible.

The display device of one embodiment of the present invention can beused in the display portion 8302. The display device including thesemiconductor device of one embodiment of the present invention has anextremely high resolution; thus, even when an image is magnified usingthe lenses 8305 as illustrated in FIG. 27E, the user does not perceivepixels, and thus a more realistic image can be displayed.

<7-3. Electronic Device 2>

Next, FIGS. 28A to 28G illustrate examples of electronic devices thatare different from those illustrated in FIGS. 27A to 27E.

Electronic devices illustrated in FIGS. 28A to 28G include a housing9000, a display portion 9001, a speaker 9003, an operation key 9005(including a power switch or an operation switch), a connection terminal9006, a sensor 9007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices in FIGS. 28A to 28G have a variety of functionssuch as a function of displaying a variety of information (e.g., a stillimage, a moving image, and a text image) on the display portion, a touchpanel function, a function of displaying a calendar, date, time, and thelike, a function of controlling processing with a variety of software(programs), a wireless communication function, a function of beingconnected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading out a program or data stored in a recording medium anddisplaying it on the display portion. Note that functions of theelectronic devices in FIGS. 28A to 28G are not limited thereto, and theelectronic devices can have a variety of functions. Although notillustrated in FIGS. 28A to 28G, the electronic devices may each have aplurality of display portions. Furthermore, the electronic devices mayeach be provided with a camera and the like to have a function of takinga still image, a function of taking a moving image, a function ofstoring the taken image in a memory medium (an external memory medium ora memory medium incorporated in the camera), a function of displayingthe taken image on the display portion, or the like.

The electronic devices in FIGS. 28A to 28G are described in detailbelow.

FIG. 28A is a perspective view illustrating a television device 9100.The television device 9100 can include the display portion 9001 having alarge screen size of, for example, 50 inches or more, or 100 inches ormore.

FIG. 28B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal 9101 can be used as asmartphone. Note that the portable information terminal 9101 may includea speaker 9003, a connection terminal 9006, a sensor 9007, or the like.The portable information terminal 9101 can display text and imageinformation on its plurality of surfaces. For example, three operationbuttons 9050 (also referred to as operation icons or simply as icons)can be displayed on one surface of the display portion 9001.Furthermore, information 9051 indicated by dashed rectangles can bedisplayed on another surface of the display portion 9001. Examples ofthe information 9051 include display indicating reception of an e-mail,a social networking service (SNS) message, or a telephone call, thetitle and sender of an e-mail or an SNS message, date, time, remainingbattery, and reception strength of an antenna. Alternatively, theoperation buttons 9050 or the like may be displayed in place of theinformation 9051.

FIG. 28C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) on theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. The user can see the display withouttaking out the portable information terminal 9102 from the pocket anddecide whether to answer the call.

FIG. 28D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, reading and editing texts, music reproduction, Internetcommunication, and a computer game. The display surface of the displayportion 9001 is curved, and display can be performed on the curveddisplay surface. The portable information terminal 9200 can employ nearfield communication conformable to a communication standard. Forexample, hands-free calling can be achieved by mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication. Moreover, the portable information terminal 9200includes the connection terminal 9006 and can perform direct datacommunication with another information terminal via a connector.Charging through the connection terminal 9006 is also possible. Notethat the charging operation may be performed by wireless power feedingwithout using the connection terminal 9006.

FIGS. 28E, 28F, and 28G are perspective views of a foldable portableinformation terminal 9201 that is opened, that is shifted from theopened state to the folded state or from the folded state to the openedstate, and that is folded, respectively. The portable informationterminal 9201 is highly portable when folded. When the portableinformation terminal 9201 is opened, a seamless large display region ishighly browsable. The display portion 9001 of the portable informationterminal 9201 is supported by three housings 9000 joined by hinges 9055.By being folded at the hinges 9055 between the two adjacent housings9000, the portable information terminal 9201 can be reversibly changedin shape from the opened state to the folded state. For example, theportable information terminal 9201 can be bent with a radius ofcurvature greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices described in this embodiment are characterized byhaving a display portion for displaying some sort of information. Notethat the semiconductor device of one embodiment of the present inventioncan also be used for an electronic device that does not have a displayportion.

At least part of this embodiment can be implemented in combination withany of the other embodiments and example described in this specificationas appropriate.

EXAMPLE

In this example, an evaluation sample including an oxide semiconductorfilm was fabricated and the sheet resistance of the evaluation samplewas measured.

<1. Structure of Evaluation Sample>

First, the structure of the evaluation sample will be described withreference to FIGS. 29A and 29B.

FIG. 29A is a top view of an evaluation sample 450, and FIG. 29B is across-sectional view taken along a dashed dotted line M-N in FIG. 29A.

The evaluation sample 450 includes a conductive film 404 a and aconductive film 404 b over a substrate 402, an insulating film 406covering the substrate 402 and the conductive films 404 a and 404 b, aninsulating film 407 over the insulating film 406, an oxide conductivefilm 409 over the insulating film 407, and an insulating film 418 overthe insulating film 407 and the oxide conductive film 409. An opening444 a reaching the conductive film 404 a and an opening 444 b reachingthe conductive film 404 b are provided in the insulating films 406 and407. In addition, an opening 446 a and an opening 446 b reaching theoxide conductive film 409 are provided in the insulating film 418. Theconductive film 404 a is connected to the oxide conductive film 409 inthe opening 444 a and the conductive film 404 b is connected to theoxide conductive film 409 in the opening 444 b.

In this example, a sample corresponding to the evaluation sample 450 inFIGS. 29A and 29B was fabricated and the sheet resistance of the oxideconductive film 409 was measured. Note that three samples (Samples A1 toA3) which were different in the composition of the oxide conductive film409 were fabricated and evaluated in this example.

The size of the oxide conductive film 409 was W/L=10 μm/1500 μm in eachof Samples A1 to A3. Note that each of Samples A1 and A3 was a sample ofone embodiment of the present invention and Sample A2 was a sample forcomparison.

<2. Fabrication Method of Samples>

Next, a fabrication method of the three samples (Samples A1 to A3) willbe described.

First, the conductive films 404 a and 404 b were formed over thesubstrate 402. A glass substrate was used as the substrate 402. As theconductive films 404 a and 404 b, a 50-nm-thick tungsten film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film were formedwith a sputtering apparatus.

Next, the insulating films 406 and 407 were formed over the substrate402 and the conductive films 404 a and 404 b. As the insulating film406, a 30-nm-thick silicon oxynitride film was formed with a PECVDapparatus. Moreover, as the insulating film 407, a 400-nm-thick siliconoxynitride film was formed with a PECVD apparatus.

Next, heat treatment was performed. The heat treatment was performed at350° C. in a nitrogen atmosphere for one hour.

Next, a resist mask was formed over the insulating film 407, and adesired region was etched to form the openings 444 a and 444 b reachingthe conductive films 404 a and 404 b, respectively. The openings 444 aand 444 b were formed with a dry etching apparatus. Note that the resistmask was removed after the openings 444 a and 444 b were formed.

Then, an oxide semiconductor film to be the oxide conductive film 409was formed over the insulating film 407 so as to cover the openings 444a and 444 b. Note that Samples A1 to A3 were different in the formationconditions of the oxide semiconductor film.

[Sample A1]

For an oxide semiconductor film of Sample A1, a 100-nm-thick In—Sn—Sioxide (hereinafter also referred to as ITSO) was formed by a sputteringmethod. Note that the composition of a target used for forming theIn—Sn—Si oxide of Sample A1 was In:Sn:Si=85:10:5 (weight ratio).

[Sample A2]

For an oxide semiconductor film of Sample A2, a 100-nm-thick In—Ga—Znoxide was formed by a sputtering method. Note that the composition of atarget used for forming the In—Ga—Zn oxide of Sample A2 wasIn:Ga:Zn=1:1:1.2 (atomic ratio).

[Sample A3]

For an oxide semiconductor film of Sample A3, a 100-nm-thick In—Ga—Znoxide was formed by a sputtering method. Note that the composition of atarget used for forming the In—Ga—Zn oxide of Sample A3 wasIn:Ga:Zn=4:2:4.1 (atomic ratio).

Next, the oxide semiconductor film was processed into an island shape,and the insulating film 418 was formed over the insulating film 407 andthe island-shaped oxide semiconductor film. As the insulating film 418,a 100-nm-thick silicon nitride film was formed with a PECVD apparatus.

Note that when the insulating film 418 is formed, hydrogen in theinsulating film 418 enters the oxide semiconductor film, so that theoxide semiconductor film serves as the oxide conductive film 409.

Next, a resist mask was formed over the insulating film 418, and adesired region was etched to form the openings 446 a and 446 b reachingthe oxide conductive film 409. The openings 446 a and 446 b were formedwith a dry etching apparatus. Note that the resist mask was removedafter the openings 446 a and 446 b were formed.

Through the above process, Samples A1 to A3 were fabricated.

<3. Evaluation of Sheet Resistance>

Then, the sheet resistances of the fabricated Samples A1 to A3 weremeasured. FIG. 30 shows measurement results of the sheet resistances ofSamples A1 to A3.

As shown in FIG. 30, the sheet resistance of Sample A1 was 1.3×10²Ω/sq., the sheet resistance of Sample A2 was 7.9×10⁷ Ω/sq., and thesheet resistance of Sample A3 was 3.4×10² Ω/sq.

As described above, the sheet resistance of Sample A1 was the lowest,followed in order by those of Sample A3 and Sample A2. The oxideconductive film 409 of each of Samples A2 and A3 was formed usingIn—Ga—Zn oxide. A comparison between Samples A2 and A3 shows that sheetresistance differs by more than five digits when the composition of theIn—Ga—Zn oxide is changed.

It is suggested that when the oxide conductive film 409 of Sample A1 orA3 is used as the oxide semiconductor film 108_2 of the transistor 100in FIGS. 1A to 1C, for example, the contact resistance between the oxidesemiconductor film 108 and the pair of electrodes (the conductive films112 a and 112 b) is lowered.

Note that excess oxygen is supplied from the insulating films 114 and116 to a region where the oxide semiconductor film 108 is not in contactwith the pair of electrodes; accordingly, when the oxide conductive film409 is used as the oxide semiconductor film 108 in FIGS. 1A to 1C, theresistance of a channel region can be sufficiently increased and thecontact resistance between the oxide semiconductor film 108 and the pairof electrodes can be sufficiently reduced.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

This application is based on Japanese Patent Application serial no.2016-094012 filed with Japan Patent Office on May 9, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a gateelectrode; an insulating film over the gate electrode; an oxidesemiconductor film over the insulating film; and a pair of electrodesover the oxide semiconductor film, wherein the oxide semiconductor filmcomprises a first oxide semiconductor film and a second oxidesemiconductor film over the first oxide semiconductor film, wherein thefirst oxide semiconductor film is In oxide or In—Zn oxide, wherein thesecond oxide semiconductor film is In-M-Zn oxide, wherein M is Al, Ga,or Y, and wherein the second oxide semiconductor film comprises a regionwhere a number of the In atoms is greater than or equal to 40% and lessthan or equal to 50% of a total number of the In atoms, the M atoms, andthe Zn atoms, and a region where a number of the M atoms is greater thanor equal to 5% and less than or equal to 30% of the total number of theIn atoms, the M atoms, and the Zn atoms.
 2. The semiconductor deviceaccording to claim 1, wherein the second oxide semiconductor filmcomprises a region whose sheet resistance is higher than or equal to1×10² Ω/square and lower than 1×10⁶ Ω/square in a region in contact withthe pair of electrodes.
 3. The semiconductor device according to claim1, wherein a peak is observed at around 2θ=31° not in the first oxidesemiconductor film but in the second oxide semiconductor film when acrystal structure of the oxide semiconductor film is measured by XRDanalysis.
 4. The semiconductor device according to claim 1, wherein thefirst oxide semiconductor film comprises a region not comprising the M.5. The semiconductor device according to claim 1, wherein when an atomicratio of the In to the M to the Zn is x:y:z and x is 4, the second oxidesemiconductor film comprises a region where y is higher than or equal to1.5 and lower than or equal to 2.5 and z is higher than or equal to 2and lower than or equal to
 4. 6. The semiconductor device according toclaim 5, wherein the atomic ratio of the In to the M to the Zn is 4:2:3or in a neighborhood of 4:2:3.
 7. The semiconductor device according toclaim 1, wherein when an atomic ratio of the In to the M to the Zn isx:y:z and x is 5, the second oxide semiconductor film comprises a regionwhere y is higher than or equal to 0.5 and lower than or equal to 1.5and z is higher than or equal to 5 and lower than or equal to
 7. 8. Thesemiconductor device according to claim 7, wherein the atomic ratio ofthe In to the M to the Zn is 5:1:6 or in a neighborhood of 5:1:6.
 9. Adisplay device comprising: the semiconductor device according to claim1; and a display element.
 10. An electronic device comprising: thedisplay device according to claim 9; and an operation key or a battery.